So I was really enjoying my first game of beta 4 on Normal difficulty with a medium map. I played maybe 8 or 9 hours total over the weekend, saving and reloading several times during different play sessions, and had no trouble at all. Then today I loaded up my game, played a few turns, went to save it, and CRASH! This is what I got:

Naturally I re-loaded the game and attempted to save it again, but it crashed again. Every time I try to save my game, the crash happens, so unfortunately my game has become unplayable at this point and I'll have to start a new one. I'm loving beta 4 so far, the game is shaping up to be everything I could ever want in a fantasy TBS. I just hope that serious, fundamental problems like this get fixed before release.
To help you guys out, I have uploaded the debug.err here: debug.err
And here's the crash dump
Last but not least, I hope you can reproduce the crash using this save game
I don't really expect anyone to reply to my post, but when you guys figure this out it would be interesting to know exactly what the problem was.