One thing to keep in mind is the presumably upcoming migration to Oracle9i compatibility. This is going to present a major break in continuity, as there will be almost no direct compatibility between the two versions. Granted, migrating will be relatively easy, but not quite automatic, so all the APMs will have to change. We've decided to re-brand the version we've put out for 9i as M(odified)ACS, so that there will be no confusion about interoperability. It will hopefully help us keep clear what should work with what.
OpenACS will also run into the same problem at some point. Though that might take place at the 5.0 marker, so it'd be easier to make a compatibility break.
All that said, I'd probably go with 4.3 or 4.4. I think the move from ACS to OpenACS is the biggest differentiator, and should be upheld in its own right. Making a big numeric jump for a characteristically differenct product seems too much like aD's Java 4.6 fiasco. So I'd say understate the version leap, but emphasize the essential technology leap in going from ACS Classic to OpenACS.
