Why would I need ten tiles for a four by four square mat? It seems to me four tiles would do. I want to put in under an air compressor to minimize noise/vibration.

For our interlocking tiles, you will lose some square footage when you interlock them. So even though our Portable Stall Mats are 2' x 2' in size when interlocked, they cover and area that is 23.13" x 23.13". Now since you will be keeping the edging on for your install, you will gain square footage back. So for your install, 4 tiles would be enough to cover the area under your air compressor.