The Alloy case Mitsy box has 2 variants

The common KM-145 [ KM-132 ] and the later updated V5M-21

The KM series has a "weak as ****" 5th gear that would split , because to get the overdrive ratio Mitsubishi had to make the gear quite small.

What Mitsubishi did to remedy this on the V5M-21 was make the driven gear larger which effectively lowered the 5th ratio.
Then they corrected the overdrive ratio by gearing up the countershaft [ cluster ] drive ratio and also lowered the 1st, 2nd,and 3rd ratios because the countershaft is now spinning faster [ giving the same corrected ratios for both gearboxes ]

The KM-145 and the later updated V5M-21 5th gears are totally interchangeable

The most probable scenario is your KM-145 has split 5th gear sometime in the past and been replaced with the larger V5M-21 5th gearset so 4th and 5th will appear to be almost the same ratio.

You can pull gearsets out of normal 2WD Mitsy L200's they are totally interchangeable , Only the mainshaft and tailhousing are different
When ever you pull one of these apart , ALWAYS replace the input bearing [ They also always fail