Jim, The model 3 had a different wing and flapperons than the 4. The 3 wing has undercamber to the bottom side of the wing for more lift but is a slower speed wing. The 3 has a shorter rudder than most 4's although the earliest model 4 has a shorter rudder too. The gross weight on a 3 is 1050 lbs. The shorter ruddered model 4 also carries the 1050 lbs gross weight. If it has a 1200 lbs gross weight and tall rudder , and no undercamber to the wing it's a 4. Also the control mixers are different for the 3 . If it's a model 3 (more complicated) you won't have to disconnect the flapperon control rods to fold the wings. There are other small diffs. but these are the most apparent. Both the 3 and 4 used Rotax 912 four stroke and 582 two stroke engines so the engine it has or is set up for will not help much.
1. Under-cambered wing. (Slower cruise speeds but high lift wing)
2. Shorter vertical stabilizer/rudder
3. Gross weight of 1050 lbs.
4. More complicated control mixer/ no disconnecting flapperon control rods to fold wings.
5. VNE of 100 mph or less (mine was 98 mph)
6. Airfoil shaped flapperons(models 1,2 & 3)
1. Flat bottomed wing. (faster cruise speeds with this wing)
2. Early 4's have shorter rudder. Later 4's have taller rudder.
3. Early 4's 1050 lbs. gross weight later 4's with tall rudders 1200 lbs.
4. Simpler control mixer. Must disconnect flapperon control rods to fold wings.
5. VNE of 120 mph or faster depending on Speedster mods and windshield thickness.
6. Symmetrical flapperons.
Jim, if I've forgotten any major easily spotted differences someone will respond.