D4S and D4R HID replacement bulbs are more environmental friendly and the Mercury-free version of the D2S and D2R bulbs.
D4S bulbs are specifically designed for projector headlamps and D4R bulbs are designed for reflectors (there is a ceramic painted part of the bulb for light control). Also, their bases differ. There are two fitment notches on the D4S bulbs which are spaced nearly twice as far apart to fit into the projector headlamp configuration. D4R notches are spaced more closely to fit into reflector headlamps.