Lazy answer, basically following AlexP: it will be as long and as serious as you wish it to be.
There are so many variables here (exactly where the meterorite hits, the nature of the geology underneath the impact, the kind of vegetation around it, secondary effects like post-impact volcanism or ice-sheet breakup, etc etc) that giving a simple answer backed up with figures is basically impractical.
That said: here's a few thoughts.
I had a quick look at the ImpactEarth calculator, which says a few useful things, but nothing about the most important stuff like dust production, which is what will really affect the climate. The energy released by your Mistastin-equivalent is something like $5*10^{20}J$, which is in the 100 gigatonne TNT-equivalent range. That's big, equivalent to a major volcanic eruption (but not a supervolcano eruption, like yellowstone, which was about 10 times more powerful).
Now, I know that meterorite impacts can't be trivially compared to volcanic impacts, but big volcanic eruptions happen more often and their impact on human society has been documented and with a bit of handwaving and squinting their scale and effects may be close enough for your needs.
There is at least one eruption which is relevant to you here: Tambora, 1815, which involved a similar release of energy and was conveniently recent. Lots of stuff has been written about it as a result... one random example: After Tambora. Take home message: everything within a few hundred km of the impact is pretty stuffed, but the environment further away isn't so badly effected. Global average temperature drops by under a degree for about a year (but the averaging masks serious local cooling).
Global effects are pretty bad for societies that rely on agriculture (see Year Without A Summer) due to reduced temperatures and significant drop in rainfall. Longer-term effects are limited, though it may take a little while for plant and animal species to recolonise badly affected areas.
Your 4Mya species might be OK, depending on how widespread they are, how far away they are from the impact, and what sort of things they eat. If they're equivalent to humans they won't even have stone tools or fire yet, so you may as well look at prehistoric animal extinction events for an idea of what might happen to them.
You might also consider looking up the Laki eruptions (recent-ish, wikipedia link) or the effects of the eruption of Thera (bronze age, more detailed and interesting paper).
Your Popigai impact, however, is a rather different scenario. It releases about $5*10^{22}J$ (I'm assuming 8km chondrite impactor, rather than a denser rocky material) so there's an order of magnitude more energy released than even the Yellowstone or Toba supervolcanos. As it happens, the Toba eruption has been linked to a human population bottleneck, though they were ancient humans rather than the neolithic/bronze age civilization you're thinking of.
The Popigai impact was linked to the Eocene–Oligocene extinction event. This should give you some idea of the severity of the impact effects and the scale of environmental disruption. (TL;DR: really, really bad).
I wouldn't hold out much hope for your species, but their extinction isn't necessarily guaranteed, especially if they're as widespread as humans are at that time.
For specific cooling figures, I can't help you, but the Chixulub event was suggested to have caused prompt reduiction of average global temperatures of ~13 degrees in the first year, with significant cooling effects lasting several years afterwards. Your popigai-ish event wouldn't be as bad, but I can't give you anything more specific than "more than 1 degree for a year, but less than 10 degrees for 10 years".
