Game Genre Warning - This isn't really a 4X game.
Sins of a Solar Empire is a real time strategy game, plain and simple. The mechanics of its battlefield borrow from the 4X mold, but it is in almost all other ways a real time strategy game.
If you come at this from a desire to play a MOO2 or Galactic Civilizations, you will be disappointed. If you come at this from a desire to play Supreme Commander or Starcraft or similar, you may like it. It is not a bad game. It is just not the game that buzz suggests it might be.
There is very little customization to be done. All sides are predetermined. All ships are predetermined. You have some latitude as far as what research to pursue, but the tech tree is very RTS (ie - "This upgrade increases the damage of all your frigates 10%" and so on). Economy is RTS, and consists mostly of harvesting resources from asteroids. There is a limited amount of planetary improvement that can be done...the primary result of which makes each planet a better "base" from which to launch attacks or gather resources.
I haven't yet played a game where diplomacy was important, but that doesn't mean it can't be. I haven't tried it yet, so I can't comment on it.
Anyway...my feelings about Sins are mixed. On the one hand, I recognize it's a solid game on its own terms. On the other, I can't help but feel I was sold a bill of goods about what it was going to be, and that lingering disappointment has tainted it for me.
Just be aware of what it is before you decide to buy.