I would say that likely your trans has an internal problem if its
sneaking out of 4th gear. Any extra play in the mainshaft bearing or wear
caused by the infamous 3rd/4th slider cracking will cause enough slop to
let 4th pop out of gear. Try the linkage first ..but if you don't find
anything obvious...smile & change or rebuild the transmission...

Daryl of AA Transaxle, Duvall, Wa.
(425) 788-4070
1-877-377-0773 (toll free)
aatransaxle.com (web site)