Why does your jeep wrangler make a grinding noise when downshifting into fourth gear from fifth?

You may have shifted into reverse by accident, I've done it a few times

Answer

You may also be having problems with the synchro rings that allow the gears to mesh without the grinding. 1st-2nd gear share a synchro and 3rd-4th gear share a synchro too. It sounds like the 3rd-4th gear synchro may be worn.