first you have to understand what is the sample space of your random experiment.
In your case you have rolled two dice. for one dice the sample space is (1,2,3,4,5,6) for two dice the sample space is the pairs of all the possible pairs you can make taking 1,2,3,4,5,6....actually (1,1),(1,2)...(1,6),(2,1),(2,2).......(6,6) . there are 36 pairs try it and then count the favourite cases you want...it is easy.
This is your first step to find the sample space.. the second step is to fine the favourite cases that your experiment yieds.....in your case you are looking for 4