Fuel is a large part of the cooling that a piston recieves. Very lean jetting can melt aluminum pistons, regardless of engine type. A four-stroke is not as sensitive to this as a two-stroke, because a two-stroke piston has less opportunity to cool, due to firing on every revolution, but it can still damage a four-stroke. Contrary to what many people believe, a four-stroke can seize the top-end just like a two-stroke. The difference is that a seized two-stroke usually just requires a new top-end, where a seized four-stroke frequently has the rod sticking through the cases somewhere.To make a long story short, lean jetting can potentially be much more catastrophic on a four-stroke, but the jetting usually has to be so lean that there are obvious signs, such as poor running and severe overheating.