I'd suggest to try something else. Although others might disagree.
3 players - Meh… No alliances, just luck, or Sol digging in. (Played 2 times)
4 players - Since Sol only have to hold 1 stronghold, it's still imbalanced. As well as the expererience wasn't so good. (Played 3 times)
5 players - I played it 3 times with 5, the result was always the same - 3 v 2, with 3 player alliance winning it in 3-5th round.
6 players - Only amount of players, I'll play this game again. Previously I didn't have enough players, now I've played 5-6 times with 6 players, it's at least for us completely different.
I agree with my local boardgame shop's owner - if you can get 6 players, get this, if not, get Game of Thrones.
P.S. Don't want to sound, that I don't like REX, with 6 players it's awesome, even my roommate, who usually doesn't like anything with >2 hours playtime, can't wait for next time.