When finding the x intercept(s) of a polynomial function, I know that I have to set it to zero. I'm just confused because my equation is

P(x) = -(1/27)x^4 + (4/27)x^3

I don't know how to solve because when it's set to zero because my exponents aren't the same. It's not like P(x) = x^2 - 4 in which case I would solve like 4 = x^2 *square root both sides* 2 = x. That is simple but how do I solve the above equation when it has a cubed root and an exponent to the fourth? Help please!