For hill climbs, I use the Transfirmer. To change between level 3 and level 4, I step up higher, as you mentioned, and I also use upper body moves (I wear 1# weighted gloves for the workout) like overhead presses.
You can also use the high step for hill climbs.
On the sprints on the rebounder, I increase intensity by lifting my knees higher, adding arm movement, or using a different move (like jacks) that is more intense.