some of the 3's were retrofitted with metal reinforcements on the wing rib/flapperon attach points so that may not be a definitive item for qualification of a 4 over a 3 unless the buyer knows what a 4 bracket looks like and a repaired 3. That was a recommended repair procedure for any flapperon attach point that was broken in an accident. The carry throughs may not be apparently bigger to a novice so I was keying in on very apparent differences but your points are spot on if the buyer is really up on his Kitfox models.
I do think though that the struts were beefed up also on the later 4's. 3/4" to 1" struts maybe. I could be remembering wrong. I am getting old.
Jim - other things to consider if deciding between a 3 or a 4 are what kind of flying you intend to do. If you want to fly with other Kitfox's and they are 4's , 5's - 7 SuperSport then the 3 will have a tougher time keeping up with them even with the 912 Rotax and a controllable prop. The model 4 will do pretty much everything the 3 can do and fly quite a bit faster in cruise. The 3 will cruise at 80 to 90 mph real well and the 4 will do 110mph pretty easily. I'm comparing with a 912 installed. The 582 will be slower on both models. The 3 gets off extremely quick with the 912 but the 4 will probably climb as quickly. I loved my model 3 but the the 4 was quite an improvement in speed with little or no sacrifice in climb and can carry more load.