before you do this decide the biggest tire you ever plan on running, you don;t want to pay for this twice.
that said i totally agree with the 4.56 or maybe even a 4.88 just to get you a little more a head of the game incase if you do jump up a tire size or get slightly heavier tires or something along those lines. play with the calc's that various websites have and compare your rpm's to see where you want to be.
i'm running my stock 4.30's on a 285 duratarc and they seem to be ok, i noticed a slight lose in power from the stock tires size but nothing i couldn't live without.
but really i think what rip was saying is there isn't really a need to regear with a 285 on most stock 4runners depending on what gears yours came from the factory with, so you may notice a improvement but not as much as you maybe expecting............