I would NOT install a Scion HU in my 4runner. Even though an aftermarket HU is a little more expensive, it is well worth it. They offer many many more features.
I would suggest checking out Crutchfield.com. They offer free shiping, they give you that insert, that you need for the right height, for free, they give you a free wiring harness, and their customer service is great. I would highly suggest them.
What are you looking for out of your new HU and what is your price range?
Edit: Don't buy from Circuit City or Best Buy, PLEASE! Most of the time they have NO idea what they are talking about. They might even mess up the install of car audio products.