Personally, I choose PHP5 for a couple of reasons.
First, it's the newest version of PHP, and while there is some breakage with older PHP scripts and some more recent large projects (I'm looking at you, Mambo), for the most part it is backwards compatible with PHP4.
Second, from a programming perspective, it is much more powerful than PHP4.
Third, at least on my server, PHP5 does seem to respond slightly faster with database-based applications than PHP4. I don't know if this is due to changes in PHP itself or changes to the database plugins, but it's measurable on my server.
Of course, your mileage may vary.