Understanding NumPy split function to extract sub-grid

So I was given a worksheet exercise as follows:

1. Given the following grid of 25 values, extract the central 3 x 3 sub-grid of 9s from the larger grid using the split() function:

   1 2 3 4 5
   1 9 9 9 5
   1 9 9 9 5
   1 9 9 9 5
   1 2 3 4 5
   

And the solution is as follows:

x = np.array([[1,2,3,4,5],
        [1, 9, 9, 9, 5],
        [1, 9, 9, 9, 5],
        [1, 9, 9, 9, 5],
        [1, 2, 3, 4, 5]])
x1, x2, x3 = np.split(x, [1, 4]) 
y1, y2, y3 = np.split(x2, [1, 4], axis = 1)
print(y2)

My question is, why is it [1, 4] in the brackets? does this refer to the element number, if so, should it not be [1, 3]?

Sorry if this seems like a very simple question - am still super new to coding!!

Thanks in advance :)

source https://stackoverflow.com/questions/74524188/understanding-numpy-split-function-to-extract-sub-grid