jmc

There are $2^4=16$ possible outcomes, since each of the 4 coins can land 2 different ways (heads or tails). There are 2 possibilities for the penny and the dime: either they're both heads or they're both tails. There are also 2 possibilities for the nickel and 2 possibilities for the quarter. So there are $2\times 2\times 2=8$ successful outcomes, and the probability of success is $\frac{8}{16}=\boxed{\frac{1}{2}}$.