Which should I try next? I'm a huge fan of MoO and MoO2.

Strengths and weaknesses of each? I've watched part of a playthough on youtube of SE4, but couldn't find any for GC1 (all the youtube vids are GC3).

Thanks for any input.

I've not played SE IV, but if you loved MoO1 and 2, you should find plenty to love about GalCiv 1. I've poured a zillion hours into it. It's pretty easy to pick up and play. In-game feedback is excellent, visual style still looks nice in 2015, and you can go pretty deep into a playthrough before the 4X micro-management becomes too burdensome... but it has plenty of tools to limit that. Basically, once you tip the scales so that there's no way to lose, you will win without having to do all the clean-up.

I absolutely love it, and can pick it up and play today and still find it fresh and entertaining.

Well, in my opinion Gal Civ is much better than Space Empires IV. The user interface is better, you have more options to make the game fit your abilities and playstyle and it doesn`t take itself too serious. Read the descriptions of new technologies and you will recognize often things you know from movies or books and often it will make you laugh.