While Lu is correct in saying that the lead/lag is predominated by Coriolis forces, I would venture to say that the form drag does have a small effect as well. In other words, if you had an infinitely rigid blade and ran it edgewise through the air there would be a slight lag due to the increased drag on the advancing side and the opposite on the retreating side. However, this is just theoretical thought exercise. Real helicopter blades are much more affected by Coriolis forces as the increased speed causes increased lift which leads to flapping...
As for all four blades on a four-bladed head being 90° from one another, are you sure Lu? I was under the impression that they are not, and think Johnson, Leishman and Prouty have diagrams indicating such. Not sure though.