OK, so maybe its not an issue and may be normal. When I start down a hill on a road that is a pretty good grade and I let off of the gas, the 4Runner shifts down into a lower gear automatically. My past vehicles (99 Taco and 99 4Runner) never did this. Is this normal?

 

That's normal. Many cars have had it for years. Honda calls their version "grade logic."

 

Also let me add that it only occurs when I put on the brakes.

 

Yes. That's normal. If you are going down a hill and start braking, it downshifts to add engine braking. This is designed to help save your brakes on long downhill grades, particularly when towing.