So, after some discussion on the #jruby IRC channel, the core team has decided to go with a 5+ strategy on trunk. The reasons for this is that almost everyone who commented on the issues advised us to move on, and the features of 5 is quite compelling. We are moving to annotations for describing Java method bindings, we will use enumerations for many values, and we will be able to use the native concurrency libraries. All of these are large wins.
Of course, we are not leaving 1.4 users completely behind. First of all, we provide a Retrotranslator/Retroweaver (we haven't decided which one yet) version, and will continue doing so. Secondly, the 1.0 branch will continue supporting Java 1.4.
This is the current state. First versions of the annotation based method bindings are already in trunk.