I recently upgraded 2 clients to 4.3.5 – both went pretty well, with minor glitches to work out. The 4.3.5 version may be the final release before the big upgrade to 5.0, which is a complete rewrite. I expect that transition to be a challenging one for most stores, so they may hang out on 4.x for a while.

Many hosting providers are starting to push users to php 5.4, as support for 5.2 and earlier is dropped. 1&1, for instance, is giving customers until April to switch (they are forced to switch in April). For many applications, 5.4 will not be a big issue, but some functions are removed in 5.4, so you should test your applications in advance, if you can.