I know it's not mainstream physics, but here I go again (it's always good to know non-standard approaches). According to this beautiful theory, quarks and leptons are not fundamental particles, and according to this model, the higher generations are excitations of the first generation. So maybe at very high energies, the constituent particles (of the quarks and leptons) can be excited to a fourth generation, while it's also plausible that at such an energy the excitation is such that no higher generations are formed but high-energy standard particles.
That there are no more than three generations was predicted in this experiment in connection with the decay of the $Z$-particle (one of the three massive particles that convey the weak force). In the model I referred to also these particles are composite though, so I think it's still an open question. Or maybe it really shows there are (in the light of this model) no more than three generations, and an eventual fourth one can't be excited at very high energies.