How to find the maximal order of a symbolic expression composed by multi vars???

X=4*c4*x^3 - 15*c5*x^2*y^2 - 15*c5*x^2*z^2 + 3*c3*x^2 ; in which c3,c4 and c5 are constants; x, y and z are variables.

so the maximal order should be 4.

Is there a function or simple way to give the maximal order ????

Thanks:)