P4 only come in the later 65 mm fork mounting as I understand.
If you're changing your fork bottoms, why not just go straight to radial calipers?
Either check ebay or speak to the uk importers,
http://www.gprdirect.co.uk/ I've used them a couple of times for parts and spares, and also a service on one of my Brembo radial m/c, and they always provide good service.
As for radial m/c, I just bought a brand new one off ebay for £145, or again check GPR.