The difference between them is 4-2 = 2
A 2 flute tool may have more chip clearing space, especially important in gummy materials. Depending on what kind of a tool it is, it may also have center cutting (ie., drilling into solid) capabilities that are better in 2 flute than in 4 flute. This is also because of chip clearing ability.
4 flute is designed to allow you to increase your feedrate, since the chip thickness per tooth is relatively independent of the number of teeth on the tool.