I've had 1 1994 SR that had the issue and my friend has a 1998 with it, It engages 4th gear really hard. I wonder if it's caused by having the ATF flushed with homebrew Dextron + friction modifier from lube shops that don't buy vehicle specific oils. Basically they buy the cheapest ATF in bulk and add the friction modifier to make it compliant with your vehicle, I don't think the jerk/hard engage hurts the clutches but it's certainly uncool. I say this because the only Montero i've had with the issue had it's trans flushed by a mom and pop lube shop and the issue developed sometime after that.
I'm thinking the torque converter doesn't unlock between 3rd and 4th gear so when it makes the final shift it will engage hard. The TC should however unlock, shift and then once you hit cruise RPM lock. I can watch the TC action on my tach in my '98 and '99 Monteros, way back in my old '94 it seemed like once it locked in 3rd it didn't unlock until i went back to 2nd or 1st. My wife drove that truck for years and she drove it really hard, i've since sold it to a friend and he's also put a ton of miles on it and a lot of hard wheeling. I don't think the issue makes the truck any less reliable just less comfortable and i think there would be an easy fix for it.