Sins of a Solar Empires does have the four x's accounted for, but a lot of other things that're typically in a 4X game, particularly a 4X game set in space, are missing or simplified.
There's no custom ships. Capships gaining experience and letting you choose abilities doesn't count, because eventually (should they survive long enough) they will all have the same four abilities. I know Civ4 doesn't have this and that's considered a 4X too, but I'm talking about
Custom ships is the big one for me, the lack of it really limits replayability. Every side has units that fill the same roles, a necessity to balance the game, but if there were a way to make custom ships then one could, instead, balance components and hulls. There is a vital choice here, though it could probably be condensed now to balancing between 5 or 6 extremes.
With custom ships it becomes less of an issue that X ship is better than Y ship, because whoever made Y can redesign it and the components/hulls would hopefully be priced so that it's fair enough anyway.
Most 4X games have identical components and hulls for all races and yet, the ability to mix and match them to fit what YOU need or how YOU think that race would design their ships, adds a major facet to gameplay and replayability. I know GalCiv II is coming out with an expansion that changes this (meaning different components and research for difference races), but they set the bar a lil differently and there's no telling yet if it'll really pay off. It might even be neutered before the expansion comes out, like the post-game 'stories' were in Dark Avatar.
Obviously a big reason we don't have custom ships is that the game is played in real-time. This would prohibit a system as in-depth as GalCiv 2's, where one can spend 15 minutes to an hour designing a ship to look and function as optimally as possible. There's surely a solution though, a happy medium, if only someone could figure it out, and no matter why a feature is cut out it doesn't change the fact that it's gone and missed.
Secondly, research is highly abbreviated, to the point where it is consistent with research in many RTSes. I don't get into lots of RTSes but I know there's several 'age up'/level-up that have about as many research options as Sins of a Solar Empire. Although, research is so expensive in this game that you must pick and choose what you really really want right up until the end, and that's good, that's more strategy.
Thirdly, and on this one it's mostly 'there' Diplomacy is a bit less vital than many 4X games I could think of, it might actually be a viable strategy allowing you to step back and let everyone else hammer away at eachother (and NOT a foolproof strategy, to its credit). Keeping an ally to the very end might demand some sacrifices in, say, Galactic Civilizations 2, but it's something that can actually last to the end. In Sins, a 'team' can win but there's no option for alliances once it begins. This might have a lot to do with a multiplayer focus where one wants to keep things fair, and there's certainly enough options mechanically where you can form 'alliances' in reality TV parlance just to ensure you and the other party will make it to the 'final round'.
Combat is certainly more determined by what you do ahead of time, and outside of combat, than the actual tactics therein. You can say the same for many RTSes, though. It's easy to run away and preserving your forces is a very good idea, that's actually a very meaningful decision that's marginalized in many RTSes. So, that's 4X-ish, you have to think about the big picture even when the fire starts.
It's a lil weak compared to an RTS, imo, you can't have a frontline and back ranks and so on while in combat since the ships just maneuver around eachother at will and attack any enemy they want. You need a highly superior force to actually deny access to an area. Some abilities are very crucial to fire off manually so they do inject some tactics in, however.
Flank and side attacks don't do anything special (though probably 90% of RTSes are the same way), though capships are most powerful offense-wise from the front and slow in turning so there is some tactic in that.
When it comes to the level of choice and the importance of those choices in combat goes, I'd give it a 7/10 compared to the best RTSes of recent years, but it does substantially if held up to /all/ RTSes that've come out recently. A lot of them are much worse in that regard
All in all I'd say it's a pretty good game, but it's not a true 4X. It is fair to say it is 4X-meets-RTS, because that doesn't mean it'd have every feature of both genres. Personally, I think it lacks the replayability of a good 4X game. I'd compare it equally to Civ4 in this regard, what it lacks in horses-vs-tanks it makes up for with actually decent combat.
I know some people might say that multiplayer is the true replayability of the game, but you know what? A good 50% of games are pretty good with the right people. There're tens of thousands of people right now playing Poker for f***'s sake.
I guess the length of time it takes to actually finish a match adds to the value here, because if you feel there's nothing new to do after three playthroughs, you've probably been at it for 20 - 40 hours and for a retail game (not Dwarf Fortress) that's considered a good deal.
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I meant that Civ4 lacked custom units too, but it's not a space 4X game so, it wasn't what I was trying to compare Sins to.
You're a necromancer ! 