K.K.
Love and Logic is a great resource. They have a website with books, DVD's, classes, etc. We started using their strategy of: "Uh, Oh, need a little room time. So sad, throw a little fit and call me when you are sweet." (you sing it to whatever is natural for you). "I'll be happy to start your timer when you are sweet (or quiet if screaming)." Once the child is quiet in the room, start the timer for one minute per year of age. Once it 'dings' we give big hugs and tell the child how much we miss her/him. No need to rehash the offense. If you are able to respond without anger (the point of singing to him the Uh Oh song), the child will be able to figure out why. It does not matter if he plays in his room either. This eliminates power struggles as well. At first my kids had to have the door shut since they would not stay, but it does not take long for them to learn that "I will be happy to leave the door open if you can stay in your room."
Great resource, hope you find it beneficial as well.
With this age also, I really leaned on the DVD "The Happiest Toddler on the Block" by Dr. Harvey Karp. Great stuff there and it really works....still does with four year olds too.