I own a 2004 Highlander and it recently began to fail to shift into 4th gear on the highway. It began a few weeks ago and only happened very infrequently but now it is doing it about 70% of the time. The transmission fluid is full, has no metal flakes, and does not smell burned. I have tried searching the internet but can't find any similar instances. Can someone please lend some insight on what is causing this? Am I doomed to need a transmission rebuild/replacement?