I am thinking of getting a 2000 mustang gt and the dealer told me that when you shift into forth it wants to slip out of gear he doesn't know what the deal is. If this is an easy fix i might buy it. What do you guys think? I am a noobie
If the car has an aftermarket shifter it's possible the shift stops aren't adjusted correctly.
If the stops are okay or if the car has a stock shifter you're probably looking at a transmission rebuild to replace worn or damaged synchros and possibly other components (e.g. bent shift fork...)