I had this situation with 3rd gear on a Honda 200x three wheeler several years ago. It was a constant humming in 3rd gear, not just when first shifting into it. Yours only makes the noise when shifting into 4th but once there it stops? Mine ended up being a chewed up gear, when then took out the corresponding gear on the other shaft. So I had to split the cases and replace two gears. A lot of time, but not hard or very much money.
Rob