If a country left the XCOM project, can they eventually come back? I didn't really have a great start to the game, and 4 of them have left. Now I have all of the others in check and the 4 that left are marked with an X.

Nope, once they are gone they are gone

@delorean99947: The ones that leave the XCOM project are gone for good. So try to win the game before you lose 4 more.

I don't think you necessarily have to restart your game, but try to get satellites over top of any remaining countries that have high panic levels (3 or more bars).

@BisonHero: @Sackmanjones: Thanks. I have satellites on the remaining countries and I was curious about it. Defiantly not restarting, really far into the game, been doing side missions to get good ships and get better gear.

Guess North Korea were never fans of the XCOM project.