I had a site on aolserver4 + beta21 until last night, when I reverted it back to 3.3 also. It was doing the same thing Andrew and Steve describe.
Oddly, while beta17 worked for Steve, it was even worse for me. It would get stuck in a loop spewing out error messages that could bring the log file up to 2 GB in a matter of minutes. Quite impressive, actually. :) It seemed to work ok otherwise but we didn't stay on it long enough to really know.
