I have only been able to get through games if there is only one AI opponent, else save corruption.

I did 

Ythril, conquest, large map, all settings normal.

The only issue was with autolocking but autosaves were working fine.

Yes.  Large map, hard AI.  Custom faction + all canon factions.

I play large maps with 3 opponents. I've completed 3 games with no corruption - 2 spell of making, 1 alliance. Am trying to achieve a quest victory but the game starts to slow down around turn 200 and becomes virtually unplayable before I can build up enough strength to take on the Dragon Quest. No instability - just slow - so I pick an easy way to end it.

Haven't had time to finish a full game, but have not experience any crashes or corrupt saves thus far.

I honestly doesn't get corrupted save-game files for some reason, but I can't be bothered taking the last few turns in my large maps, I usually am trying to check up on different strategies, bugs and features, how they work and how it is bugged.

currently working on strategies, and figuring out what I think is out of balance with the current cities

that said, I do save once each 2 hours, quit the game totally, re-enter the game and load my own game. just for the sake of memory sweeping the game, the game have some issue with not dumping memory correctly (seemingly), so this helps me.

Sincerely
~ Kongdej

I've only played one game of beta 4 so far, and I could not finish it because of the save corruption issue.  Medium map, 5 AIs, normal difficulty.  Save game got corrupted around turn 170 and I gave up on it.

Its possible.  For myself, the final building was greyed out (saying not enough prestige - though I had more than enough) so long as the first 3 towers were being built.  But as soon as they all completed, the final building unlocked, and it was possible to construct it and cast the spell as normal.  So if anything is bugged, it is the description.  My issue with it is the low costs makes it a easy way to win, and then get a terrible score, but hopefully it'll be rebalanced by the next beta.

All my games that I save end up corrupted. The only way not to have my games ruined is to not save/close the game.

I've been through and entire Large Map 8 Faction game and won through conquest on a turn after 300 (I forget specifically where it actually ended). I've also played another game to where it was pretty much a done deal, also Large but only with 5 factions. In all my time of playing (Beta 4) I've never encountered a game ending due to crashes, save corruptions, or other similar factors.

Good Poll ! ... Good idea for a Discussion Thread, StevenAus !

I got a late start on the Beta 4, so have only started (and just completed) one game. 

I played with a Custom Faction, on a Small Map, with only two opponents, on the Normal Difficulty level.  I achieved a Conquest Victory, in just under 300 turns.  I did have about 6-7 crashes during the course of this game (including some tactical battle stalls, that "automated play" resolved);    but I save often,  and I did not experience a Corrupted Save File.

For Performance reasons, I tend to turn off graphics effects such as "Particle Effects", and some of the other default features.  I favor a "simplifiction ploy"  (smaller maps, fewer opponents, and completely "fresh" exits-and-re-enters, every hour or so -- see the last paragraph of Kongdej's Reply #13 above).        I really think this may help, to reduce the issue of Corrupted Save Files.  It was a habit that I had begun, even before the Beta 4 ...

Unfortunately, I do believe that the Beta 4 is more fragile than the Beta 3.  It seems to have relatively more bugs and crashes.  However, so far they have not amounted to "game-killers" for me.  That said, I would hope that Stardock significantly revs up the bug-fixing effort ... and sooner, rather than later ...   This is NOT a good time to be discouraging the hard-core players, and the other Beta-testers, by appearing to be getting worse, rather than better!  

Seems corrupted games are an issue--glad it isn't just me.  I won my first by playing solid for about 12 hours and speeding up the laborious conquest finish by bullying the AI into surrendering.  My second game was played in smaller sessions over three days with the idea of a full clear (large map), but my plans were halted by non-loadable saves from the previous day.