What is the most likely cause of a bad bearing noise in all forward gears except fourth, also noticeable in reverse?

The presence of a bad bearing noise in all forward gears except fourth gear, as well as in reverse, most strongly points to a worn main shaft rear bearing. This type of bearing is critical for supporting the main shaft of the transmission and maintaining smooth operation. When this bearing begins to wear, it can create excessive play, leading to increased friction and noise during operation as the main shaft moves through its range of motion in the gears.

The fact that the noise is not present in fourth gear suggests that the specific load conditions and gear engagement characteristics of that gear may not place as much demand on the main shaft rear bearing compared to the other gears and reverse. Fourth gear often operates on a direct drive, reducing the load on certain components, which can explain the absence of noise when that gear is engaged.

In contrast, issues related to countershaft bearings or other components such as the reverse idler would likely produce more pervasive noise across all gears if they were faulty. Therefore, the distinct pattern of the noise being absent in fourth gear aligns closely with the characteristics of a worn main shaft rear bearing, making it the most likely culprit.