A D4 is a size of factory HID bulb. It is a newer version of D2 (S/R) bulb and does not contain mercury. The D4 lights operate on 42V while D2 require 85V. D4 and D2 bulbs should be used with corresponding ballasts. Two systems cannot be mixed.
We have HID D4R and D4S replacement bulbs available in 12 different colors. D4R Xenon lights should be installed to the reflector headlight housing since they gave a painted section. This helps to block light output to a part of the headlight housing to create a safe pattern. The D4S bulbs do not have painted sections since there is a shield inside the projector housing.The other option is an LED kit. These lights do not require ballasts that is why they should be removed firstly. This installation may require professional skills.