On my '90 CE, I had pretty much the same deal when I got it. Pressure would climb too fast and leakdown showed # 4 was the problem, though it ran fine. Found a line of corrosion across the fire ring that just looked like a scratch in the cylinder head. I made lemonade since I had always wanted to go through the top end of a 104.980 and see what I could get away with.

I doubt you need to worry about a crack, you're likely looking at a failed gasket or worst case a corroded head.

__________________

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