Yes Clive, but a semi-
dedicated server (in eUKhost's context) is a glorified
VPS Hosting and achieves what you're suggesting by using virtualisation — I have no interest in this (and nor should you if you want true reliability).
I can only suggest that if it were possible to have a true independent setup of PHP4 and 5 on my
dedicated server, then Alex P or another senior eUKhost admin would surely have suggested this to me by now ... they haven't!