data[index-1:index:4]

This expression will return a slice of the list or string "data" starting from the element at index-1 and ending at the element at index (exclusive), with a step size of 4.

For example, if "data" is a list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], and index is 5, then data[index-1:index:4] will return [4, 8].

The slice starts at index 4 (5-1) and ends at index 5 (exclusive), so it only includes the element at index 4, which is 5. The step size is 4, so it jumps over 3 elements to get to the next element to include in the slice. Therefore, the slice also includes the element at index 8.