with 4 valves you have an effectively larger inlet and outlet port for the combustion gases. thus a more efficient engine.

the surface area of four circles above the piston is greater than the surface area you can fit on the same diameter circle if you use two or three circles (makes for an interesting calculus test problem if you're the nerdy type).

Down side is a more complicated manufacturing process because of the greater number of moving parts, hence a more expensive engine.