Convex function error

I get an error if I define |x|^4 this way. Am I doing something wrong?

Error using cvx/pow_cvx (line 144)
Disciplined convex programming error:
Illegal operation: pow_abs( {convex}, {2} )

Error in cvx/pow_abs (line 13)
y = pow_cvx( x, p, ‘pow_abs’ );

Your outer pow_abs has an argument which is not affine. Hence the error.

help pow_abs

POW_POS Power of absolute value.
P must be real and greater than or equal to one.

Disciplined convex programming information:
    pow_abs(X,P) is convex and nonmonotonic, so X must be affine.
    P must be constant, and its elements must be greater than or
    equal to one. X may be complex.