What is the difference between 4-leg and 2-leg momentary tactile switch?
Most people seem to think that the 4-leg tactile switches are made that way only to add mechanical strength, but Spehro Pefhany told us in this great answer that there is another, more important reason for the 4 terminals:


  The dual shorted pins allow inexpensive single-sided boards to be used for X-Y matrices of switches without requiring jumpers. 


In other words, the additional shorted terminals are meant to be used as jumpers.



I have built one myself (see below). Those boards are a lot of fun to route.