There isn't an easy answer to this.
First thing to check, of course, is fluid level and condition. With the engine running, the fluid should come up to the appropriate crosshatch area on the dipstick. When checking the fluid, dab a bit of it from the dipstick onto a clean white shop towel and see what it looks like. Red and transparent with a chemical odor is good, brown or black with a burnt odor is very bad.
It's assumable that the 4th clutches are slipping, and the only way to find out the reason for that is to remove and disassemble the transmission. It's not going to be due to wiring or any computer issues. Any electrical symptom that would affect 4th gear operation would also affect 1st gear (one solenoid controls both ranges). If you have 1, 2, and 3, but slipping in 4, the transmission is going to need to come apart and be repaired again.
These are very healthy transmissions, failure isn't all that common. It's likely that your overhauler didn't get to the root cause during the last overhaul, despite going 40k miles. Once slippage has started, it accelerates quickly and the burned clutch material contaminates the rest of the transmission.