Excel: Calculating differences between 2 values in a large range of cells

I would be very happy to get some advice on how to, in a range of cells, calculate differences between 2 values:

  1. 20:25:37,000
  2. 20:25:36,670
  3. 20:25:36,669
  4. 20:25,36,610
  5. 20:25,36,492
  6. 20:25,36,410
  7. 20:25,36,409
  8. 20:25,36,289

I would like to use a pattern here. First I want to calculate 1-4 and then 5-8, etc…
Is there any efficient way to perform this by some formula that you can use with autofill for instance?

Much grateful for tips