The overuse of unnecessary brackets and the lack of any spacing make your expression very hard to read.
Multiplying by 1 will never change the result and will never make any difference to any error.
Your expression, without all the unnecessary brackets and with some spacing:
z = (4 - 2.1*x.^2 + x.^4/3)*x.^2 + x.*y + 4*(-1 + y.^2)*y.^2;
In my opinion much easier to read, and you can immediately see the two errors. You're doing matrix multiplication with x.^2 and y.^2 instead of element-wise multiplication. Changing the two * into .* is probably what you want:
z = (4 - 2.1*x.^2 + x.^4/3).*x.^2 + x.*y + 4*(-1 + y.^2).*y.^2;
