My interpretation of the warranty on corrosion would be no, because I inquired about it with Toyota before in regards to the metal bumper on my 1990 truck. Toyota says it's only on the body sheet metal only and it has to rust through completely (rust perforation).
However, I would still take it to Toyota, worst they can say is no. I had the rear bumper on my '05 Tacoma replaced after three months when there was rust on it. The service writer said, I needed to take it up with the service manager and he would make the call. He wasn't there, I wrote him a letter with photos (to avoid making multiple trips to the dealer to find him) and said, this isn't what I expected from a Toyota product, let alone a three month old truck. They called back and agreed replace the bumper.
What year is your 4Runner? I'll have to take a look at mine.