I bet I will end up playing both GalCiv 2 and SE5. GalCiv has no tactical combat and is more strategy/diplomacy oriented.
SE4 is more tactical/combat oriented (not that it lacks strategy or diplomacy though
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GalCiv has NO multiplayer other then scoring submissions (metaverse) while SEIV has upper hand here.
Judging from previews their successors (GalCiv 2 and SE5) will have same attributes.
So, in a nutshell, both games can be played at the same time, as they don't offer same kind of experience.