My guess would be the dogs on 4th gear are worn. You'll have to split cases, replace the two gears in that set, and put it back together.
I recently tore down a 420FM with a bad rattle. It had a bent shift fork that was dragging and making the racket. While I had it apart I noticed a little wear on 4th gear, but the owner didn't want to spend the money on a new gearset since it looked ok-ish.
Got it back together and it skipped a little if I downshifted at high RPM's from 4th to 3rd. Should have replaced the gear while I was in there. I told him to take it easy going into 3rd and 4th and it would probably hold up for awhile.
Look at posts 10 and 19 in this thread. Sounds like you need that $181 gearset which includes a new shift drum. I would guess the gearset has been upgraded due to problems in the early models and thus the new part number.
If you are at all mechanically inclined you can do it, you just have to keep track of everything you take off. Google "420 crank youtube" and a 4 part video will pop up showing you how to disassemble and reassemble that motor. I get a big piece of cardboard and put parts in groups as I remove them. Makes putting stuff back together much easier. Just put the groups in order and put them back on in reverse order.