The reason to replace all 4 senors is simple. They fail about the same time. Do the job once and that way you will not drive for a week, month, etc. and have to do it again. Sometimes it is easier to replace parts when you already have things a part. I guess you paid for an hours worth of labor even if the job took 15 minutes. I surmise you could do all 4 for that hour of labor and the only additional cost is the parts.. You will be back if you only change one.