The 4 foot depth is that depth to which historical records show that frost (frozen ground) can penetrate. In some parts of the country it's less, in some places it's more. Some folks try to get away with not going the distance and installing shallower footings. Luck may be with them and they'll never have a problem. But more often than not, frost will get under the shallow footings and lift...and then drop. And the structure sitting on those footings will move, also, and can sustain damage. If that structure is connected to another, both are damaged.
The question is two-fold. Will the local authorites allow you to show less in your design plans? Will luck be with you if you go shallow?
[This message has been edited by treebeard (edited July 10, 2003).]