What do you want with your 4x?
If you want separate tactical / strategic battles with ship design: Sword of the Stars.
If your main concern is the strategic battle and planet management (and a campaign): Galciv II.
If your main preference is RTS with a 4x flavor: Sins of a solar empire.
I've treid all 3, spent quite a bit of time and all 3, and they are all good games - the question of which is best answered by what you want in your game.
Personally I have a strong preference for designing ships and fighting the tactical battles manually. As such, Sword of the Stars is probably my favourite choice. However, GalCiv II is much better in other areas.
Bottom line, direct answer: If you want a 4x space game, Gal Civ II is worth it.