Very very doubtful that they would work - I can almost guarantee they won't.
First of all, the tracks are different, so a replay on a GT3 track will have difficulty working on a GT4 track.
Secondly, what if the GT3 replay uses a car that isn't in GT4?...
Thirdly, and most importantly, I'm sure that when the reply is saved, it doesn't save a copy of the race - it saves a copy of the user inputs ie: 30seconds flat out throttle, brakes, turn left... but obviously much more detailed. When you play the replay, it then uses your saved inputs to recreate the cars actions on the track. This makes saving a lot smaller, simpler and faster. Because the two games are more than likely coded differently (much differently I'd imagine) recreating a replay would be very difficult.
Hope this helps...
Does anyone know if this is how GT actually saves it's replays? This is just how I
think it works, but I've got some good evidence to back it up...
