While it is true that many of the better quality underpinners will often succeed in stacking as many as four wedges, it is worth considering the amount of force required to push that forth wedge and the three wedges in front of it into the moulding.
As an ex-design engineer, I am only too well aware that all the extra force is also acting upon any parts within the underpinner and accelerating the wear of some of these parts. Not only that, but in many cases if the underpinner is pneumatically operated, then the higher level of air pressure required to accomplish this feat gives the pneumatic components in the system a pretty good hammering as well.
Be nice to your underpinner and it will last longer than if you hamer it just because you can.
While we are discussing this subject, I remembered a not very good picture i posted in another thread showing ten wedges double stacked in five positions with the aid of a laser printed paper rule stuck in place on the top plate of my CS-88. in many cases a spread of double stacked wedges will keep the corner joints strong and tight without resorting to stacking more wedges further into the moulding. Here is a link to the thread:
http://theframersforum.com/viewtopic.ph ... ges#p80427