It's definitely related to the steam update. I have two machines with steam, I ran the update on my primary (Win7/64-bit) machine and Avernum 4 stopped working.
I then copied the steam install from my secondary machine (everything except the steamapps folder which contains the games themselves) and Avernum 4 works again.
Avernum 4 works with steam client:
Built: April 18 2012, at 10:17:32
Steam API: v012
Steam package versions: 1868 / 1868
Avernum 4 does not work with steam client:
Built: May 15 2012, at 10:21:05
Steam API: v012
Steam package versions: 1869 / 1869
There were no changes at all made to the Avernum 4 install during this test, so that presumably rules out an issue due to a change on Spiderweb's side (though they may still be stuck holding the bag to work around the crash).