You can shift 3rd to 4th on a steep hill but it is not a good idea because you will loose power/speed and could even start to roll backward and stall the engine. 4th gear is designed for level road driving at higher speeds. 3rd is for stepper hills or getting to sufficient speed so to shift to 4th. 2nd gear is for steeper hills and so on.
Downshift a gear until you have sufficient power without overevving engine.
Downshift to the next lower gear. If it is very steep, go down another gear. Make sure you don't over-rev the engine though.
When going down a steep grade especially if you are pulling a load.
Shift down in to a lower gear.
When climbing a steep incline. The automatic transmission will always try to engage a high gear.
steep means you are going higher
You should change down to a lower gear, 3rd gear or if the grade is very steep 2nd gear.
select a lower gear
The purpose of the automatic transmission is that is shifts for you. You'll want to lock your transmission into your ideal gear for going up and down steep inclines to prevent it from downshifting, but, other than that, you typically let the computer do the shifting for you.
Always use overdrive except when towing or going down a steep grade.