[avmich]

You can technically imagine two legs with really wide feet, allowing some perpendicular stability. I wonder if one-leg lander could be imagined. 3-legged landing scheme was used in Surveyors, first American automatic Moon landers, and was surely considered for Appolo LEMs, but rejected. So there could be additional, secondary reasons when choosing the number of legs.

[timschmidt]

The cost of one additional leg is pretty inexpensive for the redundancy it provides for the other three.

[chipsa]

4 legs have no additional redundancy over 3. One leg failing will still result in the booster tipping over. They do push the maximum angle of tipping before your CG is no longer supported out farther though.

[kevin_thibedeau]

Useful for when you land on the rim of a crater like Apollo 12.