I think it's so underrated because of the early days. The first episode especially is a really bad representation of the rest of the show
But yeah, after a few episodes it became brilliant imo, definitely hugely underrated. Season 2 is one of my favourite seasons of TV ever.
As for the season 4 finale,
Spoiler:
I did like it but I wish we got more of a look at the effects of Praimfaya on those who didn't make it into the bunker. Even though they were pretty much all randoms, just seeing them in those final moments as the death wave approached I think would have been really effective. Also certain characters like Abby etc not appearing at all was awkward.
I was sure Echo was gonna die so I'm glad she didn't, I loathed her earlier in the season but she really grew on me in the last few episodes and her being part of the group who went to space is a really interesting dynamic.
Strange to think this was the first time Monty and Murphy have really interacted since they've both been around since day 1. And possibly the most unexpected budding bromance ever