The Vanagon manual transmissions don't have a method for "keeping" 4th gear
engaged.  The shifter arm MUST be free to move slightly forward and back
when the shifting lever is in 4th gear.  If not, the gear will be only
partially engaged and will wear VERY quickly.  Just resting your hand on the
shifter lever habitually when driving can cause the problem.  The gearset if
it is no longer within tolerance will not remain engaged.  Some loafdrivers
have used a "bungee" to keep 4th gear engaged.  Works as a stop gap measure
perhaps for thousands of miles.  When I had a transmission installed, the
installer failed to check this vital clearance, wore out 4th gear in ~1600
miles on a 1200$ rebuild.  The builder, although it was clearly not his
fault, honored his warranty for parts and built it again at a substantial
discount.  Jeff Field, at Transaxle Engineering can give you details on how
the failure might occur.  You can contact him at 818.998.2739.  A short chat
will probably be well worth your time in analyzing the problem.

Pensioner (40K+ on this rebuild and no problems)