I'm not sure what "image crawler" is, but I guess it's some sort of web robot.
Developing web robots is a very complex topic, and I've done it a bit. I've always used PHP5, because the PHP5 DOM class contains a HTML parser which makes link extraction, etc, much easier.
Image functions are available in PHP4 too, provided it was built with them.
For any new project I'd recommend PHP5 over PHP4, definitely. Unless you have a really good reason not to, use PHP5.
Mark
