I would suspect that you have one needle bearing missing in between the input shaft and the output shaft That or the front spring clip under the synchro tabs is broken or missing I have had tranny that would jump out of 2nd and 3rd gear but never fourth either way it is time to take it out and very carefully disassemble it, take your time and pay close attention to what is what before starting and during disassembly. EDIT, One other possiblility that can cause the trans to not stay in 4th gear is the pilot bushing That can and does cause a vibration when you rev the motor up also in high gear