What have you tried?

Let a = 1000x + 100y + 10z + w where x, y, z, w ∈ {0, ..., 9}, i.e., xyyw = 4 * wyzx. (Sometimes a line is written over xyzw to show that letters stand for individual digits of a single number and to distinguish this expression from a product.)

The following will get you started. There are only two possible values for w. Why? One can easily see (without considering other digits) that the first of those values is impossible. The second one passes that test and determines x. Now, similar to the case of w, there are only two options for z. Consider both options; in each one you already know three digits.

P.S. Please don't double-post.