Forum Moderators: coopster
Look at software options and what is most commonly needed and used, then decide which is the best for you to choose. There are a bunch of great functions and the differences for OOP that make 5 the better choice if possible.
Another thing is to look at what needs 4 to run and figure out why, all the functions in 4 are available in 5 but not the other way around. A bunch of the software that specifies 4 may just not have been tested on 5 or might be relying on something archaic that could easily be changed.
PHP 5 and the integrated Zend Engine 2 have greatly improved PHP's performance and capabilities, but great care has been taken to break as little existing code as possible. So migrating your code from PHP 4 to 5 should be very easy. Most existing PHP 4 code should be ready to run without changes, but you should still know about the few differences [us2.php.net] and take care to test your code before switching versions in production environments.
I would use PHP 5 if I were you :)