I often get inquiries asking why Herbie's Audio Lab recommends four Tenderfeet under a component instead of three. The question of four versus three footers has been going on forever and probably will go on forevermore. In some applications, three is better, in others, four.
The ideas of using three footers because "three points defines a plane," to eliminate wobble, or for better balancing are basically for rigid, noncompliant footers like cones, where you'll almost always end up with one "short" footer if using more than three. That's only because surfaces are almost never perfectly parallel. Many cones, bearings, and other rigid footers introduce some audible coloration, so minimal usage (three instead of four) is often better to help maintain sonic neutrality (at the cost of better vibration-control). There's oftentimes nothing wrong with using three footers, but reasons of "wobble" or "balancing" are misconceptions when considering compliant footers.
I recommend four Tenderfeet under a component because they often give an audibly better result than three. They work in intimate contact with the component chassis in such a way that more contact area is usually better than less. Tenderfeet have just a little "squish" to them to compensate for slightly uneven surfaces (for grossly uneven surfaces, they can be shimmed with Post-It Pad paper or other material). Using four Tenderfeet gives you more versatility with placement, balancing and leveling; better lateral stability; and more evenly divides the physical areas of the component for vibration control.
Different results can often be obtained by adjusting the positions of Tenderfeet. Sometimes you'll get a bit more of a clean sound with Tenderfeet placed at the very edges or corners of the component where the chassis is most rigid and strong. Sometimes, a little farther away from the edges brings out the most preferrable result. Or, a non-symetrical array may work best. Because there is no "one-size-fits-all" rule for footer placement, experimentation is needed to find an ideal arrangement. Most often, however, you'll get satisfactory results with Tenderfeet placed simply where they seem right.
Sometimes a fifth Tenderfoot adds additional benefit--placed under a CD player under where the motor is mounted, near the middle of a component with very large breadth and depth, or midway along the heavy side of an amp where the transformers are.
SteveHerbie's Audio Lab