Solved by ptrblck in post #2 
              
              
                The tutorial explains it as: 

F_XL is reshaped to form F̂_hat_XL, a KxN matrix, where K is the number of feature maps at layer L and N is the length of any vectorized feature map F^k_XL. 


It doesn’t seem to be working and I’m not sure, what this operation calculates in your case: 
a, b, c, d = 2,…