How to extract N elements every M elements from an array?

Suppose I have a numpy array [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], How do I take 4 elements every 8 elements). Here is the expected result:

a -> [1,2,3,4, 9,10,11,12]b -> [5,6,7,8, 13,14,15,16]

My array has hundreds of elements. I went through the numpy array documentation but I never succeeded to perform this computation other then a loop which is very slow.

EDIT:The array can have up to 3 interleave sub-array of 4 elements

4 elt sample0, 4 elt sample 1, 4 elt  sample2, 4 elt sample0, 4 elt sample 1, 4 elt sample2, 4 elt sample0, 4 elt sample 1, 4 elt sample2 ...

My array has 499875840 elements !