The vast majority of players that actually play a lot and were not able to abuse the bug are at 4 or less?
In any case, I did not say there was one. You however stated that there was no cap, like it was a fact.
You don't know that, thought it seems likely at this point that the chances of getting a legendary after 4 are so small, most people will never get one.
Once you hit 4, you're better off rerolling a new character as epic gear and AP is much easier to get.