Damn 400??? (shoulda been a mechanic...sigh) Dude a couple things could be wrong. First of course, bad tranny, but I doubt it. If I remember right, 4th is directly on the shaft. That could mean the input shaft is not seated or the pilot bearing is shot. I would think pilot bearing before I thought bad tranny in this case. IN ANY CASE, take that $hit back to the mechanic and tell them the problem in case it was their fault. You need to get to the bottom of it, whether it was the mechanic's fault or the manufacturer's fault so you can get it fixed.