oh bu55er.....I thought I was the only person to spot those

Yes, you will find that essentially it is the same bike.....and the engines are interchangeable as a complete lump.
(there may be wiring loom differences as grey imports don't have a side stand switch, but do have an annoying red light on the dash that comes on at 80 kph....a japanese regulation)
You may find the rear sprocket is a different number of teeth as , if i remember correctly the 400 is geared down a bit.
the 400 may only have a single front disc and caliper
good luck
SteveL