Thanks everyone for the input but nobody was quite right-on. I wanted to modify my development environment under WinXp/IIS, not the host environment running Apache. I actually found the answer in the PHP documentation, though it was a bit vague in places, it did cover everything I needed to do to enable the '.php4' suffix on my system - some IIS parameters, a couple of registry tweaks. "When all else fails, read the manual."
I might investigate the possibility of Earthlink using '.php' instead of '.php4' but having looked at a number of threads on dealing with Earthlink tech support, they appear to be clueless about anything regarding PHP, even the need to use the .php4 suffix. (I have tried using '.php' - doesn't work.)
Thanks again!