There may be debris that block the gear to shift to 4th gear.
You can try to flush the tranmission. Buy flushing fluid at Walmart or auto part shops. Follow instructions.
Jack up the front driving wheels, pour flushing fluid in mine, idle the engine, shift back and forth all 4 speeds to make sure all gears are flushed.
Then drain the fluid, add back in the 4 or 6 quarts of new fluid to fill the pan.
Remove the transmission fluid return line from the radiator to the xmission, stuck it into a large, clear can, like a gallon milk bottle.
Again idle engine while shifting through all 4 speed. Have someone keep adding fluid into the dip stick hole. Watch the flushed fluid in the bottle. After adding 4 to 6 quarts of fluid, the drained fluid should look clear, fresh and red. Then xmission is completely flushed. Hopefully the debris is flushed out and unstuck your 4th gear.
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