hardtimes,
The only reference to 4 degrees in the other post is what FlatheadTed wrote:
"When the advance plate is set on a KR Wilson its around about one notch above the center line.that will give you roughly the 4 Deg advance."
This is a bit of a head-scratcher.
First, trusting that the marks on the index plate EXACTLY relate to EXACT timing degree points would probably be a mistake.
There are in fact 7 marks. And, by design, the middle one may be 0 degrees of initial advance. However, the actual history of an individual distributor may make this undependable and that makes using some other method, such as the KR Wilson, to set the initial advance a good idea.
After doing lots of distributors on a fixture such as the KRW, you'll probably find most end up aligned with the 3rd mark from the bottom ... which I'd call one below the center ... Ted has that Southern Hemisphere thing going and in New Zealand it might be one above the center.
If one were going to "guess" at which mark to use, most would favor a notch more of advance, meaning below the "center" mark.
Back to 31chevy's question. Each notch should be 4 degrees ... crankshaft degrees. The "two per mark" answer would be correct if you're talking distributor degrees. The crankshaft rotates twice for each distributor rotation.