1999 Gearbox help

My reverse, 1st, 2nd, 3rd and 5th gears all work perfectly fine. I can shift into 4th gear however after about 200 metres the gear stick moves into neutral for no apparent reason. Does anyone know what the problem is and if possible what a mechanic will charge me to get it fixed.

Hi, This is from a retired ford tech, You have a worn synchro in the transaxle, If you can live with the problem that's ok, If not, Its time for a rebuild, Current prices in the midwest at a union shop are about $99 per hour, About 8+ hours labor + parts.....Good Luck, Clay....