Per Bill Taylor, for a 4" span he recommends 1/8 forward. I believe he is talking about total span, not cut to cut. For every 1/8th your span increases, your thumb pitch adjusts by 1/16th, so if your total span is 4 1/4, your thumb pitch should be 0. His thumb exceptions are if you have a very dry or short thumb you should add 1/8th forward. If you have a very damp or long thumb you should add 1/8th reverse.
These are good starting points and then determine what is best for you. Take care....JS