The transmission is arranged in such a way that one shaft (input) uses gears to connect to an intermediate shaft then gears drive the output shaft to the transfercase and driveshaft. 4th gear is 1:1 ratio which does not use the gearing of the intermediate shaft. I have read that a transmission noise that shuts up in 4th gear is typical of input bearing excessive wear due to lack of oil.

I forget the specifics as to why the noise quiets in 4th gear, something about a load not placed on the bearing as its supported by other bearings or something.

My trans had this problem when a related problem of oil flowing into the t-case deprived it of oil at high speed. I eventually junked the trans. many would agree that you may be looking at that since input shaft bearing replacement is a difficult procedure requireing extensive (and expensive) trans rebuild expertise.