I just moved a large ACS 4 Classic site to OpenACS 4 as an experiment with no problem. There are some minor
differences in acs-messaging and acs-mail, but I think those could be replaced with their ACS 4 Classic versions if
someone didn't want to do the minor hacking necessary to use our slightly modified stuff.
If you've kept track of what those hacks are, it mightn't be much of a problem to make our standard classic stuff play well with your setup.
OpenACS 4 incorporates a bunch of bug fixes and some (though not many) important performance enhancements,
especially in the area of permissions checking.
At least some of which we've incorporated into the classic version, but I'd be interested to see what else you guys have cooked up beyond what I've seen on the bboards. I have a few more ideas brewing myself, but I haven't had time to test them all out.
If there's one thing we could probably all use a great answer for it's the permissions checks.
