Slicing Array of four matrices
It's not really clear what RM and M are based on your description.
Is M the ndarray containing all 4 images, and RM the 2x2 array for a given pixel containing the data from the 4 images? 
You can put the 4 images into the same ndarray so it has shape (4,N,M) and then reshape slices.
For example, to get the (0,0) entry you would do A[:,0,0] to get the 4 pixels, and then reshape it to get a 2x2 array.
I've got an array of 4 images, each image lets say is NxM (all images share this same size)
(I'm implementing a Harris Corner detector by the way.)
Now I made a matrix M = ([Ix^2, Ixy],[Ixy, Iy^2]).reshape(2,2)
and now I'd like to compute my response. 
which is usually Det(RM) - k*(trace(RM)**2) 
RM being a 2x2 Matrix each point in this matrix is derived from the same coordinate location for each image in M.
How can I slice M to create RM? 
In other words how can I slice the Matrix M to create a smaller matrix 2x2 RM for every pixel in the NxM images?
For example the first RM matrix should be a 2x2 matrix taking the 0,0 coordinate from each image in M.