You'll be fine in Act 4 at your current level. The quickest way to lvl up is to get into a game with 4-8 people and join their party. You'll get a ton of exp points this way. You can also join a game and have them beat Diablo for you and then get into Act 5. Do the same thing in Act 5 in which you get into a game with 4-8 people and join their party. You'll be level 45 in no time.
AB