Maybe Stardock just doesn't have any more machines running XP? That's a mighty old OS now.
I played the first Sins and liked it but it just made me want to go play Gal Civ 2 more. Sins is a hybrid 4X/RTS game. Saying it's a 4X game with a few RTS elements would be like saying Deus Ex is an RPG with a few shooter elements. I've never been able to get into RTS games so I guess that's why I could never quite warm up to Sins.
Gal Civ 2 is solid 4X gaming. There's lots of races plus great flexibility in customizing races (your own and enemy races). You also have huge flexibility in designing how your ships look. One thing I really like is the ability to take over other planets via superior cultural influence. The big minus, though, is that it's all strategy. The tactical battles are completely hands off.
Sword of the Stars 1 (not 2… at least not quite yet) trades the ability to customize races with races that use very different modes of transport. One race will have the typical free movement you normally see in these games, another follows warp tunnels, another builds gates that will transport instantly - once they finally get built, and so on. It also switches into something of a mini-RTS when there's a tactical battle.
I liked both Civ 4 and Civ 5. Civ 5 has more radical ideas. It's the only 4X I know where it isn't always to your advantage to get bigger. (Though I guess we saw some of that in Civ 4.)
Oh, and Endless Space is hanging around out there, too. That's kind of a 4X-lite, which is something the industry could certainly use. Great game for people who've never played a 4X game before.