You are correct about 4 stroking being caused by a rich mixture, however, several things can cause this, only one of which is the main jet.
If the air filter/airbox is restrictive, then the faster you go, the more air the engine demands and the lower the pressure in the intake causing a rich mixture.
Sometimes, when you remove this restriction, the mixture will lean out at lower rpm and you have to go up a jet size or two.
Another common problem is, for want of a better name, "double dipping". At higher rpm, the intake airstream will reverse direction as the intake port closes, this causes the air, which has picked up gas while going forward, to pick up more gas from the carb as it travels backward and then still more as it goes forward again into the engine.
If you reduce the main jet size to cure this one, then you are in danger of severely leaning out the mixture at lower RPM, such as going up hill at full throttle. The result being an overheated engine recieving a minimum amount of lube.
This is why plug chops under different conditions are so important.