Thanks Brian.
I just read threads about flaring but "flaring" seems to be a temporary slide/rev between shifts. My transmission totally disengages when it shifts to 4th gear.
When I put it to D it shifts fine up to 3rd and just revs to high rpm when it shifts to 4th. 4th gear does not engage and all you hear suddenly is the high engine rev as if it's in neutral. I have to shift to S to keep up to speed. When I again shift to D at 50 mph it just revs to high rpm as if I'm in neutral. It seems like just I lost my 4th gear.
I just talked to my mechanic on the phone and says it could be a clutch disk problem. I'm hoping it's just a vacuum problem.