Leave it in D4 for the climb. The tranny computer will shift down whenever it wants to. It doesn't need your permission.

If you happen to be at some unhappy combination of speed & slope, it might shift back & forth constantly, which is annoying. THEN you can shift to D3 to make it stop. But most likely you'll never have to do that.

OTOH, you can use D3 when you're going DOWN hill, for some engine braking. That can prevent your brakes from overheating if you're in the mountains.