[Erax]

Hello, welcome to the forums. I only played the SE3 demo for a while, but the main difference I noticed when I started playing SE4 were the planets. You can colonize any atmosphere, but those with your native atmosphere ('breathables') are best. The others will have a white 'domed colony' icon when colonized, which restricts population size and number of facilities. There is a facility that changes a planet's atmosphere, but it takes time.

You've probably noticed that colony tech is now for planet types, not atmospheres.

I'll let someone who played SE3 more than I did fill you in on the rest.