Go here: http://www.eurospares.co.uk/partsListing.asp?M=1&Mo=675&A=1&B=39757&S=&ID4=1615154# click on "will this fit my car?"
I am fairly certain the rotors are the same, but the calipers definitely are. The brake hoses may be a different length between the GTB/GTS and the gt4 though.
Dunno, but if you are looking for an upgrade the the boys at QV in Ascot do an interesting little kit for GT4s using modified 355 calipers & disks.....