In general, if you have n trials and the probability of success is p, you would expect to have np successes in the long run.
E(X) = np
So if you flip 4 coins where the probability of heads is ½, the expected value is 4*½ = 2 heads.
E(X) = 2 heads
P.S. That doesn't mean you'll always get 2 heads. It means if you did this over and over and over, the number of heads would tend to average out to the expected value of 2 heads. To make this clearer, if you had *5* coins, the expected value is 2.5 heads. But obviously you would never be able to flip exactly 2½ heads because there is no such thing as half a head. Instead, if you were to repeat this process hundreds of times and average the number of heads each time, you'd find it averages out to about 2.5 heads.