I went with the 4" but that had more to do with desired adjustment height of threaded screw in conjunction with my steel base. From a weight standpoint its overkill. My mill weighs ~1350 lbs, plus steel base & table accessories etc. call it maybe 1500lbs? The 4" diameter say they are good for 1200 lbs alone. They seem to do a good job of vibration dampening & conforming to my slightly sloped floor, but I also don't have anything else to compare against. Some guys on forum have made some good feet from other materials including hockey pucks.