Perhaps it's based on the 600? Since the GSR isn't a no holds barred hyperbike, the weight penalty of sizing down a 600 class motor to meet the road requirements of a 400 class would be acceptable... much like the JDM's SV400 being based off the SV650.
Just a thought... OTOH, maybe Suzuki has plans for another GSXR400 that'll use a hotter version of thie GSR4's lump.