This is something I’ve been pondering over for a while now, and I’m leaning towards forking for jME 4. I expect that jME 4 is likely to have some pretty major architectural changes, so I think it’s likely to widely diverge from the existing codebase. At some point having two wildly divergent branches in the same repo seems counterproductive. I’m not hard set in that opinion, and of course it will be some time before we know just how extensive major changes are likely to be.
This is rather off-topic for this thread though, so if we wish to discuss it further we should take the conversation to a different thread (possibly this one).