how can I fix the insufficient error in fitsread command?

I am using this loop to read 57 fits file which are about 4gig.

path='C:\Users\Kianfar\Documents\Thesis\DATA';
files=dir(path);  
a=zeros(5,936,936,5,4);
as=zeros(5,936,936);

        for i=3:7
            a{i}=fitsread(strcat(path,'\',files(i).name));
                for j=1:5
                    as{i}=as{i}+a{i}(:,:,j,1);
                end
        end

but it says:

Error using ==> double Out of memory. Type HELP MEMORY for your options.

Error in ==> fitsread>readprimary at 161 data = double(data)*bscale+bzero;

Error in ==> fitsread at 43 data = readprimary(info,raw);

• _how can I fix this error?

p.s. the code works for 5 data, but not more than that._ *