I don't know about the MTE 1800D. It shouldn't be that hard to figure it out, though. You will certainly have a range selector for high/medium/low (Or whatever combination that machine has), and a PTO shifter. The couple photographs I could find seemed to show a speed select shifter on the left fender, and the PTO shifter on the left side of the transmission tunnel. There will be two other shifters, one for the transmission range, and the other for front drive. Once you locate those, the one that doesn't change the tractor's speed must be the 4WD lever. Post some pictures for us and we can perhaps give more specific advice.