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There are a couple of things it could be. Some are easy, some are "transmission replacement time". Here are some things to check:

1) Is the shifter link worn? This is the link on the passenger side from the shifter to the gear selector lever, and the holes at the ends should be the same size as the shafts that it fits on. If the holes get ovaled or the shafts worn, it might not allow 4th to be fully engaged, so it will jump out. Climb under the car while an "assistant" changes gears, and make sure there is not a bunch of play on this link. If the holes are worn, you can remove this link and either get a replacement, or get a machine shop to weld up the holes and drill new round ones. If the shafts get worn, it requires the tranny to be disassembled and the shafts built up.

The link on the driver side is adjustable (selector link) but as long as it actually engages each gear (and reverse) it should be OK.

2) Are the bolts that attach the tranny to the bellhousing tight? These are often not properly tightened after a clutch job, since the top bolts are VERY hard to get a wrench on while the engine is in the car. You have to use a cut-down 15 mm wrench, since a full size wrench and even a socket on multiple universal joints won't reach it. I find it is easier to just drop the entire engine/transmission, but that is easier said than done. But loose bolts will cause mis-alignment, which will cause jumping out of gear.

3) A worn pilot bearing (in the recess of the crankshaft, behind the clutch asssembly) can cause mis-alignment and jumping out of gear. The repair required is essentially the same as replacing the clutch, with the added step of replacing the pilot bearing. If the shift link is OK, you are dropping the tranny anyway, so it is easy to have a look at the pilot bearing.

The rest of the causes are usually some type of internal damage to the transmission. An experienced transmission mechanic (or a patient home mechanic with reasonable skills and lots of tools) can quite easily fix most problems, but they can get expensive. Here is a list that the factory manual provides:

1) End play in main drive gear (bearing retainer loose or broken, loose or worn bearings on main drive gear and mainshaft)

2) Worn clutch teeth on main drive gear and/or worn clutch teeth on synchronizer sleeve

3) Worn or broken blocking rings

4) Bent mainshaft

Good luck, and tell us how you make out.
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