Your problem specification is incomplete. Let me respond by first asking a few questions.
Why do you think you should use powers of X and Z? Are those powers prespecified? Are X and Z nonnegative? If the powers are not prespecified integer values, then you could run into problems if the algorithm which is used to fit the model tries to raise a negative value to a noninteger power.
Do you want to consider polynomial functions of X and Z? The model below is representative of a polynomial model:
Y = a1(X) + a2(X**2) + a3(X**3) + b1(Z) + b2(Z**2) + c
Finally (for now, anyway), what motivates the model that you want to fit?