2 x 2 scanning mask of an image

can anyone help me with the code of using a 2 x 2 scanning mask of an image? m stuck with the loop. and also how do i feed the pixels of that mask as inputs into a fuzzy inference system... please help.. this is the code: the FIS system ed2 takes 4 inputs and produces 1 output.

a=imread('Lenna.png'); g=rgb2gray(a); [m,n]=size(g); t=readfis('ed2.fis'); i=2;j=2; while i~=m while j~=n for k=1:1:2

            p4=g(i,j);
            p1=g(i-1,j-1);
            p2=g(i,j-1);
            p3=g(i-1,j);
            o=evalfis([p1 p2 p3 p4],t);
        end
        j=j+1;
    end
    i=i+1;
end

and the error is: The first input must be a defined DOUBLE matrix

Error in ==> evalfis at 84 [output,IRR,ORR,ARR] = evalfismex(input, fis, numofpoints);

Error in ==> ed2x2 at 14 o=evalfis([p1 p2 p3 p4],t);