Perhaps you should figure out where you want your story to be on the Mohs Scale of Science Fiction Hardness.
The "harder" you want it to be, the more you need to read what follows.
In the early 20th century geologists discovered that the Earth was billions of years old. They also discovered fossils of Earth lifeforms hundreds of millions and eventually billions of years old. Those fossils showed that the surface temperatures on Earth remained fairly constant for billions of years.
Astronomers and physicists calculated how long the Sun could shine with energy from gravitational contraction, and they found that the Sun could shine for only tens of millions of years, a tiny fraction of the age of the Earth, and thus of the Sun, according to geologiests.
So there were strong disagreenments about the age of the Earth between geologists and phycists and astronomers. I have read that actually led to punches at one scientific conference.
Then in the 1920s, 30s, and 40s, astronomers nand uclear physicists worked out the nuclear fusion processes which actually powered stars. They began to calculate how long a star of a specific spectral class would shine as a fairly steady main sequence star before becoming a red giant and then a white dwarf, killing all life on its planets.
At the same time geologists established that Earth was about 4.6 billion years old, and found evidence that there had been life on Earth for billions of years, and that some lifeforms on Earth had eventually produced the oxygen rich atmosphere which humans and all large multicelled lifeforms on eArth need to life. They found that Earth only had an atmopshere breathable for humans during the last few hundred million years, a small part of the history of Earth.
Combining those facts, it became obvious to the more scienficially literate science fiction writers that only main sequence stars belonging to some spectral classes could have planets that could possibly - other factors being right - support human life or be otherwise interesting for most types of science fiction stories set on other planets of other stars.
So in Robert A. Heinlein's juvenile science fiction novels Starman Jones (1953) and Time for the Stars (1956) it was mentioned that main sequence spectral class G stars would be most suitable for having planets habitable for humans and for lifeforms with similar environmental requirements.
Stephen H. Dole published Habitable Planets for Man (1964) discussing the scientific factors necessary for a planet to be habitable for humans or for lifeforms which have similar requirements.
On pages 67 to 72, he discussed the properties necessary for a star to have a habitable planet, calculating lower and upper limits of mass and luminosity.
On page 68 Dole calculated the upper limit of stellar mass for for a star to possibly have a habitable planet is aobut 1.4 stellar masses, a spectral class F2V star.
And if you ignore such calculaitons you may run the risk of being considered 60 or 70 years behind the times when it comes to selecting the spectral classes of stars in your fictional star system.
The only way for a writer to get around the spectral class limitations for habitable planets is to claim in the story that a highly advanced civilization terraformed a young and uninhabitable planet in the system of a young star to make it just as habitable as planet billions of years older would become. Or maybe those aliens took an older planet that was already habitable from another and older star system and mmoved it to the younger star system of the story.
Your idea that your planet could have been naurally torn from orbit in one solar ssytem, traveled through interstellar space for countless millions of years, and then been recapured into orbit in your solar system is very dubious. That would be a very rare sequence of events.
If it is possible in your story for a super advanced civilization to deliberately move a planet form one star system to another, that should happen countess millions of times more often than a planet accidentially moving from one star system to another.
And if a planet accidentially and naturally moves from one solar system to another, it should freeze up for the countless millions of years it will take to do so, killing all life on the planet. But if a superadvanced civilization can deliberately move a planet from one star ystem to another, they should have no problem keeping the planet warm and lighted while it crosses interstellar space, thus keeping life alive on the planet.
Or you can simply put your planet in orbit around a star of the proper spectral type.
Or if you don't care how hard or soft your story is, you can use any type of star you want.
A planet habitable for humans can not be at just any distance from its star. Each star has a specific luminosity, and each specific luminosity has a different sized circumstellar habitable zone, where an otherwise suitable planet would have the proper temperatures for having liquid water on its surface.
Calculating the size of the circumsteller habitable zone around a star is easy in Theory. Just take the luminosoity of the star compared to that of the Sun and scale the habitable zone of the Sun up or down.
But the size of the circumstellar habitable zone of the Sun is not known with certainty.
Here is a link to a list of recent estimates of the inner or outer edges or both of the Sun's circumstellar habitable zone. Note how much they differ.
One way to make certain a fictional planet will be within the habitable zone of its star is to calculate the exact distance from that star that a plent would receive exactly as much radiation from the star as Earth receives from the Sun, which I call the Earth Equivalent Distance or EED.
The answer by user177107 to this question:
Has a table with a list of data about differnt spectral classes of stars. The date includes the disances of planetary orbits at the EED for each listed type of star.
Note that according to Dole, the most massive star capableof having a habitable planet would be an F2V class star, with a mass about 1.4 times that of the Sun. Such a star would have an EED of 2.236 AU according to the table.
Right now, I'm getting the feeling that I should place the planet from 2.something to 3 AU away from a smaller (2 solar masses) star, and 6-8.5 AU away from the larger (4 solar masses) star.
I assume you want the smaller star with two solar masses to more or less orbit around the larger star with 4 solar masses, and for the planet to orbit around the smaller star with 2 solar massess.
According to the table of stars I mentioned earlier, a star with 2.05 solar masses would be a class A2V star, and a luminosity 21.243 times that of the Sun, and its EED would be at 4.611 AU. Thus at a distance of 2 to 3 AU the planet would receive consideriably more radiation from the smaller star than Earth gets from the Sun, and so should be considerably hotter than Earth despite having a higher albedo. Your temperature of about minus 20 degrees C seems improbably cold even without considering the radiation the planet would get from the larger and more distant star.
Vega is a class A0Va star with a mass of about 2.135 solar masses, and a luminosity 40.12 times that of the Sun, and thus its EED should be at about 6.334 AU.
A spectral class B8V star would have a mass of 3.8 solar masses, and a B7V star would have a mass of 4.45 solar masses. So your larger star with "(4 solar masses)" would be between a B7V and a B8v star, and closer to the B8V star.
18 Tauri is listed as a B8V class star, and has a mass of 3.34 solar masses and a luminosity of about 160 times the luminosity of the Sun. Thus its EED should be at about 12.649 AU.
Thus a planet that is 6 to 8.5 AU from a star with 4 solar masses should be hotter than Earth, no matter how high its albedo, even if it wasn't also even closer to the smaller star in the system and also heated up by that smaller star.
Your planet needs to have had a stable orbit for a long time.
A planet in a binary star system can have one of two types of orbit. Exoplanets with both types of orbits have been found.
A circumbinary or P-Type orbit is when the planet orbits around both of the stars, which are much closer to each other than the planet is to them.
An S-type obit is when the planet orbits one of the stars and the other star is much farther from the planet. Your description with one star several AU farther away from the planet the plane tthan the other star indicates it is an S-Type orbit.
In non-circumbinary planets, if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.5
In your example, with the planet orbiting one star at a distance of 2 to 3 AU, and the other star being 6 to 8.5 AU distant, the ratio of distances is 2 to 4.25, less than the 5.0 minimum ratio for orbital stabiity.
According to this list, the closest known orbital distance between two stars with a planet orbiting one of them in an S-Type orbit is about 12 to 17 AU. Since the planet orbits one of the stars at a distance of about 0.7 AU, the distance ratio is about 17 to 24.
You desire that the average surface temperature of the planet is about minus 20 degrees C. That is below the freezing point of water, so there should be no liquid water on the planet and no liquid water using lifeforms. Thus photsynthesis whould never have produced an oxygen atmosphere on the planet, and it should be uninhabitable for humans.
So the colonists you mention in your question should never have colonized the planet.
Maybe they colonized the planet to mine it, and they live in pressurized buildings like in a moon base, and work in mines deep underground or in massive excavating equipment in open pit mines. Thus they should be protected from ultra violent ultraviolet rays by the roofs and walls of their buildings and by their vehicles, and by rock and/or ice when they work in the mines.
Since the air is unbreathable they will have to wear breathing gear outside, and since the climate is so cold they will have to wear very warm clothing outside. So I guess that they might as well wear protective gear when outside which covers their entire bodies to keep them warm and supply oxygen, and which also prevents all ultraviolet radiation from reaching their skin.
The Earth's atmosphere keeps a lot of ultraviolet radiation from reaching the lower atmospher and the ground. Since Earth has a breathable atmosphere wiht a lot of molecular oxygen (02), some of that is converted by various processes into ozone (03), and some of that ozene forms the ozone lawyer in the upper atmosphere which blocks a lot of ultraviolet radiation from reaching the surface.
So if your planet is not so cold that it has no native life and no oxygen in the atmosphere, but instead has native life and a breathable oxygen rich atmospehere, like most planets which are colonized by humans in science fiction stories, it should have an ozone layer which block a lot of ultraviolet rays from reaching he surface.
And possibly you could find a way to change the composition of the atmosphere to block out much more ultraviolet radiation and make the planetary surface safe for your colonists.