Hi,
By design, the transmission downshifts when you release the throttle and slow down. However, you normally wouldn't feel it.
Have you checked the fluid to make sure it is both clean and full?
https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid
Also, the idea that it only does it in 4th gear is odd. It could even be a failing universal joint or center bearing. In pic 1 below, I highlighted the center bearing and both universal joints. You can inspect those for evidence of wear. Also, pic 2 shows the carrier bearing. That can certainly cause what you described.
The bearing in the carrier is mounted in rubber, so check to see if anything is worn or loose.
Let me know what you find.
Take care,
Joe
See pics below.
Images (Click to make bigger)
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Tuesday, December 13th, 2022 AT 9:27 PM