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Hello, my name is Bobby. Thank you for the question. If it is hitting high RPMS in all of the other forward gears then the transmission is failing. 4th gear uses a different lock up so that is why it is not slipping when placed into 4th. You are going to need to have the transmission rebuilt or replaced. You can verify this by draining the fluid and checking it. If it is really dirty or has debris in it then you definitely know that this is the problem.