A polynomial of four terms is sometimes called a quadrinomial, but there's really no need for such words. That's because the number of terms in a polynomial is not important. There are special techniques for factoring quadratic trinomials, which is why that term is used; sometimes a quadrinomial is worth mentioning for the same reason, but beyond that, a polynomial is just a polynomial, and the degree is more important than how many terms happen to be non-zero.