Assuming that you mean a model of the form
Y = A + Bx^2 + C x^4
Simple is to download my polyfitn from the file exchange, which can allow you to specify a specific model.
As easy for this very specific case is to use polyfit, on the transformed quadratic model:
Y = A + Bu + C u^2
where you define u as x^2. Get that from:
p=polyfit(x.^2,y,2);
