I imagine the way it works it out looks something like this:

#!javascript
function ts(n) {
  return n - (n - n % 4) || 4;
}

// ts(17) = 1
// ts(5) = 1
// ts(4) = 4
// ts(6) = 2
// ...

This makes looking for tabs in pull requests when there should be spaces more difficult, as 2 spaces in is harder to spot that 4 spaces out.

It'd be great to have this just set the tab-size to whatever you enter - I have a feeling this may have been how the change from the default tab size of 8 before the the new 4 was implemented though.