Gears change without a command from the driver.

Your transmission probably has electric solenoids controlling the various gears. You need to have them checked out for proper operation especially the one responsible for your 4th gear..

run the fault codes and look for a faulty torque converter clutch solenoid /servo and a problem TCM unit. A service if not done in the last 60.000 miles would also help.

new spider gears or possibly u-joints in your rear drive shaft. Or it could just be a cheap rear i have a jeep and its done it since i first bought it.

ignition control module

Is the check engine light illuminated? If so have the vehicle scanned for codes. Depending on what the codes indicate a repair is necessitated.
Sounds like a shift solenoid(s) may be the problem or the ECU. Unless you have training in transmission repair I suggest bringing this to a Transmission repair shop ( preferably an ASE Shop). The ECU are known to eat upt the gears on these models. Good luck

How many miles do have on the vehicle? Has it been put on a Scanner? It may need to be flushed just to rule out any internal Trans. damage.Try the flushing first and then persue the other possibilities..

Well, there are a number of questions that come to mind from a routine maintenance perspective.
Seeing that it is a 1991, has the transmission ever been serviced. A complete oil change out and filter replacement should be done every 50K miles or so.

Let me know how many miles you have and if you plan on keeping the vehicle.
Regards,

how many miles have you driven the car sence it came out of storage? you said it sat for 6 years it could be rust on fly wheel or pressure plate, that would make it shift like you said, because the clutch plate is either sticking to the fly wheel or pressure plate. when it shifts hard that means the clutch is not releacing all the way most likely because of the rust. i would just drive it and see if it comes out of it. it shoud not take long maybe 50 to 100 miles more or less. also i would chg. trans oil, setting that long moisture could have got in there, you know what they say about oil and water. also if it has a cable for clutch chk. see if it's hanging up to. setting that long rust becomes a big factor. hope this helps.

Replace the transmission box bushes

Check the idle speed.