Forum OpenACS Q&A: Response to Planning a migration from ACS 3.4.x to OpenACS?
I'm sure Ben would tell you this, but he (and the rest of the team and community) are pretty adamant about upgrade paths. We have made them for the 3.x series, we'll continue to do so for the 4.x series and on.
There's no OpenACS 5 formally planned yet, but with the experience we're gaining in OACS4, we sure are starting to form the ideas of what needs to be done for. But right now, all our focus is on getting 4 out the door.