Apply function to image window.

Hi all,

After reading an image and converting it to gray I want to apply a simple function I created

est = findest(segment, base)

to lets say 4x4 windows in it.

So I want to take the small 4x4 image segment, apply the function to it and move on traversing the whole image. What is the best and fastest way of doing that in matlab (I suppose using two for loops is not the ideal method)