The bigger issues is that PHP4 isn't very good (I'm not saying that PHP5 is perfect, but it is significantly better). It shipped in an insecure manner, and a lot of programs rely on that behaviour to run.
An example would be the "Magic Quote GPC" runtime option, that auto escaped " and ' etc. People used this (and still do) as the sole defence against SQL-Injection.
The other thing is that PHP4 wasn't really developed with forward compatibility in mind. One of the changes between 4 and 5 is that the variable $this is reserved - now obviously, good programmers would never call anything "this", they'd assign it a descriptive name. Sadly, PHPs ease of use has meant that everyone with a text editor and a <? echo "hello world" ?> * script calls themselves a programmer. So I've seen ever this issue cause problems.
Getting rid of PHP4 is a very good idea.
J
* - short tags was another strange idea, try having short tags turned on and using "include()" on an xml file (Of course, you shouldn't use include for that purpose...)