At this point, VaultWiki 4.0 is relatively feature-locked. You may have noticed in the last few releases, the biggest thing we added was the ability to turn things on or off. There are a few things we want to get in before gold, like some more options for moderators, but nothing major.
I think we will be moving to RC after one or 2 more Gammas. The features that we add after that will be minor until we get a stable release (a new setting here or there, maybe a new search filter, etc).
Prior to 3.0, I'm sure you noticed that new features typically did get added on second point releases: 2.0 had namespaces, 2.1 had Syntax Manager and Namespace Manager, 2.2 had SEO/custom URLs, 2.3 had an updated installer, 2.5 had a lot of improvements to namespace design.
But we got to 3.0, and I think we never increased the second point because around the time that 3.0 was gold, we announced what we were working on for 3.1. We didn't want to release a 3.1 that didn't include everything we had announced, so any new features that we wanted to release early had to be added to 3.0. But it turned out what we had announced was a much bigger project than we thought it would be, and we never saw a version 3.1 anyway. Even so, we did feature lock 3.0 around 3.0.8 and told users they would have to wait for version 4 to see certain things, so that flux you refer to was only in the beginning of its life cycle.
I don't think we'll be following that route again. VaultWiki 4 is so abstract, there's very little I can see us needing/wanting to rewrite at all, and most of it is small stuff that could be done within a single third point release, if we wanted to. So I don't see us wanting to do anything so big it would be VaultWiki 5, at least not for a very long time. For VaultWiki 4, we didn't want to release a Gold version before we had support for the big 3 forums (vB, XF, IPB), but IPB4 doesn't have a release date in sight, and we don't want to keep our version 4 in testing forever.
That said, we are already working out our plans for 4.1. We don't want to say much about it now. There's plenty of new stuff in 4.0 that people can get excited about.