Once you get to the higher difficulties (hell and inferno mainly) your gear is going to matter a LOT. It'll get to the point where one hit will kill you, the gear can turn this into 3-4 if you're lucky
Regardless, of difficulty level, you can force the monsters to respawn by restarting the selected quest, so if you decide you want to farm Act IV, you just select quest one, farm, and reselect it again when you wish to restart. When you start nightmare mode, you will have to start the game from scratch. The Risen in Act I were taking 3-4 hits from my level 34 Barb when I started, the final demons in Act IV on normal were taking about the same, so yes, the enemies do scale. But on the plus side, so does loot!
EDIT beaten to it it would seem