I am a recent conver to frameworks and PHP frameworks in particular.
After looking at many I settled on CakePHP (http://cakephp.org/).
1. It runs on PHP 4 and 5 (but my web host doesn't offer PHP 5 so I need 4 support)
2. I found it the easiest to understand and get working with
3. It has features like Ajax support built in that I needed
4. It has a fantastic developer community that will help you solve any problem that you hit. Ultimately this is worth its weight in gold.
CakePHP may not be the best framework out there (I have no idea as I haven't done a head to head test), but I _can_ say that it works, and works well. My websites are easier to develop and easier to maintain.
In my opinion, if you need PHP 4 support, then CakePHP is worth a look.