It is most likely that I will end up doing some work with ExtJS 3, but ExtJS 4, even in it's beta state, is giving me a much better idea of where it is going.
Especially when it comes to things like extending GridPanel in it's new form, trying to retro fit plugins and features like auto column widths and filters.
At the moment, ExtJS is a learning exercise for me. I've been told it has a steep learning curve, but once you've got that down, it's smooth sailing. Given the choice, I'd rather hit that curve on how ExtJS will be looking going forward, rather than how it once looked. I hope Sencha make the migration process as smooth as possible, but the next best thing is not having to do a migration in the first place.