This looks like an arithmetic sequence - remember that in an arithmetic sequence, a common difference is added (or subtracted) each time. Here, we're adding 4 each time. To find the sum of the first n terms of an arithmetic sequence, we use:

.

Also, where d is the common difference.

Combining these, along with the fact that we know , the sum, and d, the common difference, is 4, gives:

.

Now, solve for n; you'll get a quadratic which factors, although the quadratic formula is probably the easier way to go.