It is not necessarily under WOT either. It can simply be trying to pick up speed to merge in, and it does not want to go into 4th. It is a bit disconcerting when you merge in front of someone - but can't accelerate after that.
It is not a consistent problem - just something that occurs from time to time.
Well its good to know that its not only me. I wonder if it has something to do with that skip shift thing that setup on our cars. Like if we are at a certain RPM it wont let us go into 4th, but wants us to put it into 5th instead. Honestly I don't know. I was able to grab 4th gear once though out of my 3 runs.