Of course, I've just missed the simplest solution of them all :doh:
Replace the 4-pin connector on your current PSU with the 8-pin from the last one. Just cut the new one off and tape (or if you're really fancy, use one of those connector yokes with the screws. No idea of the name :shy
the one from the old PSU on. Should do the trick
Are you sure that there aren't two 4-pins on the new one though? On my PSU it had an 8-pin, and you had to split it in half for the 4-pin on my mobo. Maybe it's the opposite case on yours?