Yes, it's possible, but debugging your code to run with register_globals off and upgrading it to run on newer versions is a good thing. There are reasons that newer versions exist.
That said, just ahve 2 php directories. php4.2.3 and php4.3.4. Rename whichever you want to currently use to php so apache can find it.
Note: the php.ini file will probly need changed when you switch as well.
This is actually what I do on my laptop because of a certain bug in 4.3.x that breaks some code, but then there's a certain bug in 4.2.x that breaks some other code. Gotta love it.