I just finished a complete rebuild on a 4l60e in a friends 4th gen, I've built several of these with success but I'm not a professional by any means. all seems well after 100 miles except for 1 issue. driving in 4th with the converter locked cruising it will do a 4-3 shift out of no where. looking at the data it isn't being commanded to do so. after doing the un-commanded shift it will shift back to 4th as it should until doing it again. any ideas? thanks for any help

 

All the 4th gear issues I have seen on this forum, came down to 3 problems:
1. The PCM isn't commanding 4th gear due to a code, overheating or engine sensor problem (TPS or MAF).
2. The 4th gear servo piston is cracked, has bad seals, or was installed backwards.
3. Valve body problem. The OP replaced the VB and unfortunately never figured out how to fix the VB.

Are you logging with e.g. HP Tuners to verify that the PCM isn't commanding the 4->3 downshift?

 

Are you sure its not lockup that coming off. With a high stall it would seem like a 4-3 shift. I cant really see how you could get a random 4-3 shift with no command hydraulically.