ATF Change and 4rth to 5th gear change Issue

Normal especially after a long hill. The 5 speed auto trans is pretty smart and hangs in 4th like that to reduce hunting. It will stay in that gear even going downhill for awhile after you crest the hill if you stay in the gas cus it goes, ok there must be another hill coming up.

If its hanging in a gear lower than what you want (like in the situation you describe where its hanging in 4th lockup and you want it in 5th) you can force an upshift by moving the gear selector from D to 4 back to D (you can do it quick).

Same method works if the truck downshifts to engine brake when you are off the gas going down a hill. The trans has the programming to, under certain circumstances, downshift on its own all the way to 3rd gear going down a hill without any throttle being applied. You can go up 1 gear at a time by doing D to 4 back to D, or go straight to coasting in 5th by going D to N back to D.