Thats a very interesting idea Mike and perhaps one worth serious consideration.

I would point out though that while that modification may be more EASY to implement than a common offday every 2 weeks it may not be more REALISTIC.

If you look at MLB most of the time the 5th starter gets started it is because of an off day. By making your proposed modification it would make it so ace starters could go every 4th day whether there is an offday or not which does not exactly mirror real life. In real life about 98%+ of the time the ace will only go every 5 days but in the case of off days the 5th day might be the 4th game.

Just my $0.02. You have a very good idea, although the off day is more REALISTIC but maybe not as FEASIBLE.