4th gear is actually the torque converter locking in at 1:1 ratio for fuel economy.this doesnt happen till it reaches a certain temp. 150 or something. homers got good idea. i thought that also.seems to be common.happened to me.

so are you getting up to normal operating temps?

maybe also its not going into "fourth gear" cause of slow speed. check all connections under hood.maybe (carefully) clean mass air flow sensor,just spray with cleaner,also air temp sensor. maybe check o2 sensors.
just throwin some ideas out there.make sure no vacume leaks.any codes poppin up?

good luck keep us posted

p.s. i am no professional,take my advice with a grain of salt,and get a second opinion,and of course wait to see what axe and gary say. <img src="/forums/images/graemlins/cheers.gif" alt="" />