I have posted the size of galaxies here:     https://forums.galciv3.com/463642/page/1/#3538947

If you get an idea on smaller maps what seems right to you, then you can scale up the number of factions to larger sizes using this table.

In your specific case, 4 AI players is far too few.    i suggested in my post that if 8 was right for Huge, that you would need about 60 for insane, but I picked the 8 out of mid air as an example.   It may not be far off though, I have played with 26 AIs on Excessive and it seemed about right.   You probably want a few more than eight on HUGE.    But it is a very personal choice as to how quickly you want to interact with the AI in your game,  you just have to experiment.    

Thanks for the quick responses!  I've been used to playing large games in Distant Worlds, and GalCiv3 (currently) doesn't make it easy to determine how many players one should have based on map size, whereas DW offers lots of flexibility in this area (both in terms of logistics and scaling travel time/research/etc).

I'm going to restart my game and max out player count!

You are all playing wrong..... (Just kidding)

Obviously you need to set planets to Common but then set habitability of both regular and extreme to RARE! Play on EXCESSIVE, get a total including you of 13 AI, (13 tribes get it?). Abundant minors and RARE Pirates. 

You will average about 12-18 planets per faction but you have to search them out and finding them feels like Xmas all over. Also you will play a good 50 to 75 turns opening up the map and will likely only meet Arrr Pirates (even on RARE). 

You will have a good amount of time to get into the tech trees and can choose a path which you feel suits  your play, Diplomacy, Culture, WAR, or Technology. 

My comfort zone (from years of playing Immense maps on GC II) is about 50 planets per major faction.  Near as I can tell, an Insane A/A/A game has around 1500 planets.  Give or take.  That's quite a bit under GC II generation rate (I should expect 2000 planets if it were the same).

So, for me, I'd want 30 factions or so on an I/A/A/A game. 40 to 50 if there were a comparable generation rate as GC II.  But it also depends on your expansion rate, their expansion rate, and a whole host of other factors.

However, since production is radically different in GC III than GC II, that might allow for more factions to comforably be placed on "less" planets.  Say 25 instead of 50.  Then I'd want twice as many factions for a 'normal paced' game.  It's hard for me to say in some respects because I haven't had a chance to really explore the mid-game or anything past it on very large maps.  First because of a nasty graphics bug that kept killing my games past a certain point.  And now because of the patches coming out I keep starting over.