To extract the coefficients use the "fit" function....
[fitobject,gof,output]=fit([x, y], z, 'poly44');
fitobject.p00, fitobject.p10, fitobject.p01, etc contain all of the coefficients. So,
Coeffs = [fitobject.p00, fitobject.p10, fitobject.p01,...
is your vector of coefficients.
For more info type
[Edit: First post was a solution for curve fitting, then I realised you were surface fitting.]