It sounds likely that you are on the last few miles before a replacement or rebuild. When you have a problem with only one gear, adjstments are not likely to help as the adjustments provided affect all gears. With the mileage on the car I'd suspect that the clutch pack for 4th is worn out. When they are worn it takes more fluid pressure to hold what is left of the clutches. When the fluid gets hot it thins and doesn't hold as much pressure, to the point that it doesn't hold the gear. It's going to be money either way so I'd suggest to have it gone through by a competent trans tech sooner rather than later. The longer you continue to drive it as is, the more likely that the internals will suffer more damage and cost more to fix.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff