I think that having a prehensile tail is very useful but would actually argue against the extra set of arms as they would be very complicated for them to use properly what with all the criss-crossing of limbs.
I think a better course of action would be to make them more like monkeys, able to grip with their feet so they have the same number of limbs as us yet are able to hold more. They could hold tools or apply force (to wrench or other somesuch) with their feet, and use their hands for fine motor applications while holding themselves up with their tail.
If you look at the muscle structure of monkeys you may find more desirable traits as well, for instance monkeys have a few muscles/tendons that we either don't have or don't really use that improves their grip strength with their "less important" fingers. Improved grip strength seems like a useful feature.
Also: Gecko style fingers/toes. Heck yeah. It would make them amazing at sticking to walls and ceilings without having to use their tail and give them great grip on tools and small things, you know, like those tiny screws that dont want to come off the table unless you wet your finger or use a magnet?
If they regularly hold their whole bodyweight for extended periods with their tail it would probably be thicker than a monkey tail, I am imagining something more akin to a reptile tail in shape (thicker at the base, tapering at the end for more finesse) but that is covered in skin, like a rat tail, for good grip.
Alright, here me out on this one. Whales. Whales are mammals, they need air, but they can also hold their breath for a long time and their lungs don't strain under the pressure of the deep water. These creatures would still start losing all of the moisture in their cells in a total breach unless they were covered in a weird carapace due to the vacuum of space, but if they have very good lung capacity (or maybe more efficient oxygen usage, such as being able to purposefully slow their heart rate) they will be able to deal with very low pressure and low oxygen environments for periods of time. Ex: if there was a breach and the ship lost its atmosphere in the decompression but then an airlock closed off that section so its no longer a complete vacuum in the rest of the ship, even if it is missing the oxygen.
Its hard to give them survivability in a vacuum without making them really weird bug people.
When there is little or no gravity having boring human eyes can be very inconvenient. Anything could float into them, if there is liquid the surface tension of the liquid could make it stick all up in your face, and in space there isnt always a lot of light. Sure there are stars all over but you arent guaranteed one of them is close enough to give you good light.
I think that giving them a second transparent eyelid would help them deal with low gravity gunk not getting in their eyes, and making them have larger pupils/eyeballs would allow them to see in the theoretically lower light conditions of space.