The Toyota 4Runner will allow you to put on larger tires without worrying about rubbing the fenders. According to online forums, the highest you can go is 285/65R18. Still, you will definitely need to adjust the suspension and maybe do some fender roll.
If you are serious about putting larger flotation tires on your 4Runner to make it a serious off-road machine, I suggest going for a suspension lifting kit. The suspension lift will allow for much larger wheels, but it will also make your 4Runner much better on off-road trails.
With that said, there are some things you need to have in mind. For instance, tires with larger sidewalls will impact the handling and overall stability of your SUV and also lead to longer braking distances. Moreover, they will increase the stress on the drivetrain components, such as axles, differentials, and the transmission.
Furthermore, putting larger tires will result in speedometer inaccuracies, which will be especially noticeable at higher speeds. Finally, larger tires equal worse fuel economy on the already thirsty 4Runner.