For the majority of your immediate future in PHP programming, there really won't be all too much difference between the way PHP5 handles things compared to PHP4. In general, as with most new versions of software, the changes are additions -- new things built above and beyond what PHP4 could do (the Object Oriented changes being most significant for many people).

My advice would just be to learn using PHP5, since that's the current version. That way, it'll save you having to re-learn and/or adapt to the changes (eg, changes in the order of arguments for functions or new ways of doing certain things).

Again, perhaps I'm repeating myself a little too much but in reality there isn't all too much of a drastic change between PHP4 and 5, for the most part, and that's certainly the case for more introductory level things.