What is firmer? A 3 foot chair or a 4 foot chair?
Have you noticed that a 4-foot chair often goes awry? This does not happen with a 3 foot chair, which will always be firmer. This is because three unaligned points will always determine a plane. When we have a chair with 4 feet, we have four points that can determine up to four planes. As you can rest on either of them, the chair can be "dancing". In this case, the solution would be to place a shim on one foot so that it is contained in the same plane as the others.