The fourth clutch is slipping.

1. Rebuild the clutch.
2. Replace the piston seals.

The trimmer valve is sticking.

1. Replace or rebuild the control valve body assembly.
2. Inspect the trimmer valve.

The seal rings on the center support assembly hub are broken.

Replace the seal rings.

The seal rings of the clutch piston are cut.

Replace the seal rings.

The check balls are not seating in the rotating clutch housings.

Inspect the check balls for contamination and proper operation.