I picked up a very cheap 400/4 engine years ago as the guys in the breakers saw my 500/4 and said they had an engine that 'might fit that' in the back of a shed. I bought it home and decided it was way too different to fit, from what I rember it looked possible for a 400 engine to go in a 500 with a lot of work but the other way would not very likey be viable. If someone really wanted to it might be possible - many years ago I worked on a project fitting a cr500 engine into a Cagiva mito 125 leaving the bike looking very close to standard - might be possible with enough skill, time, rule bending etc but not worth it.