Here is a list of possibilities for no fourth gear. The first question is what condition is the fluid in? Does the RPM's raise without forward vehicle movement when fourth is attempted? Also is there a check engine light.
Slipping or No Fourth Gear
DEFINITION: A large engine RPM increase is noted in fourth gear, Drive range, with little or no vehicle forward speed increase. First, second, and third gear operation are normal. Shifts into fourth gear are soft or do not occur. Fourth gear ratio is 0.705:1.
Automatic Transmission Fluid Level
Low fluid level
Transmission Main Line Pressure
Low transmission main line pressure
Oil Filter Assembly
" The oil filter assembly (100) is missing, plugged, or damaged.
" The oil filter seal assembly (101) is missing or is damaged.
Control Valve Body Assembly
" The 3-4 shift valve (362) or the 4-3 manual downshift valve (360) is stuck or binding.
" Debris is found in the passages of the control valve body (301) or of the control valve body spacer plate and gasket assembly (396)
Case Cover Assembly
Distortion or damage occurs at the manual valve (404) or at the case cover (401), causing PRN oil leak.
Driven Sprocket Support Assembly
A plugged third clutch oil passage in the driven sprocket support assembly (609)
Fourth Clutch Assembly
" The clutch plates (500-502) are worn, damaged, or misassembled.
" A missing or not seated fourth clutch spring retaining ring (601)
" A damaged spring assembly (602) or piston (603)
" Worn or damaged fourth clutch piston seals (604, 605)
Third Clutch Sprag Assembly
" The sprag (720) was misassembled or backwards on the inner race (661).
" A damaged third clutch sprag assembly (653, 661, 719-721)
Thursday, August 8th, 2019 AT 10:21 AM