Technically, you can make mods for GalCiv too... SE4 modding is more powerful and far easier though.
GalCiv does have better AI than SE4, but the rest of the game is poorly developed.
GalCiv combat is atrocious. Attack score, defense score, hit points, no designing, no fleets, nothing. Ships fight each other one on one. Compare this to SE4 combat where fleets actually fight each other, you can design your ships, ships have deep designs, etc.
GalCiv systems suck. A handful of planets all in one square. Compare this to SE4 systems where you can have dozens of objects floating around, ranging from planets to multiple stars to asteroids to storms.
GalCiv diplomacy is a bit better designed than SE4, and its AI actually uses it decently. GalCiv is only single player though, so who cares?
Playing against the AI gets boring after a few games. Human opponents are far more satisfying.
GalCiv technology is just weird. Most techs give a 1% bonus to resource production, or combat bonus, or something like that. Compare this to SE4 techs where you actually get brand new components to use. Research armor and shields to improve the defense of your ships, how you choose to improve it. No magic universal upgrades, you get to choose how much armor and shielding is on your ships. Same with weapons and other components.
GalCiv colonies are poorly implemented. You can build every facility on every planet. SE4 actually has limits to the space on a planet based on size.
GalCiv ground combat is even worse than SE4. You just load civilians on transports and they are magically troops. In SE4, at least you have to design and build your troops.
GalCiv race customization is a joke. You can only play as humans, which is needless to say boring. You can only make a few minor tweaks. In SE4, you can play as any race (even add your own), and you can completely customize its strengths and weaknesses.
GalCiv does have a basic government system, which is interesting. However, like the rest of the game, it is rather lackluster and lacking in any sort of depth.
GalCiv has too many mechanics hidden away from the user. SE4 is better suited for strategizing.
Ok that's probably enough for now...