since Pre- nuptials are designed to protect the interest of people who are about to enter a contract-- i don't see any problem with changing them four years after the fact-- and still calling them prenuptials.

(but if it were me, after 4 years? I would wonder why are you making the terms of divorce harder? and who would agree to that? and if easier? why bother? if I have a contractual obligation to give you X, and then in a divorce settlement give you 2 or 3 times X-- are you going to take me to court and sue me because i have over paid you?)