A third degree polynomial must have an x3 term. In general, such a polynomial would have 4 terms in the form:
ax3 + bx2 + cx + d
where d is a constant. If you must have a constant term of 4, then d = 4. If you have only three terms instead of four, then either the x2 or the x term will be missing. So it is either:
ax3 + bx2 + 4
or
ax3 + cx + 4
To determine the values of a, b, and/or c, we would need more information than you have provided.