Every Toyota model uses the same auto-dimming mirror. The only difference is in the wiring. For example, if you were to purchase a Camry without the auto-dimming mirror, the wiring is already installed for the mirror and it is basically plug and play. All you would have to purchase is the mirror through Toyota. On the 4Runner, you actually have to wire everything from scractch because there is no prewiring installed from the factory. Purchasing the mirror through Toyota would not serve you any benefit.
The Gentex mirror that I bought from brandsport was cheaper than any of the mirrors sold through Toyota. I think I paid around $60 for it. It is the same exact mirror as on the all Toyota models.