There seems to be some confusion about H4 HID kits,
1- the Slim is in regards to the Ballast not the bulb.
2- there are 4 kinds of H4 HID bulbs:
A- The old style with Low beam HID & high beam Halogen.
B- The single intensity H4, has one setting & is usually low beam.
C- The Bixenon H4 this one has Hi & low beams, this is accomplished using a solenoid that moves the bulb against a shield to switch between Hi/Low but in reality the bulb intensity & light output is the same.
D- Dual Bixenon, this bulb is actually two bulbs together with two ballasts one for hi & one for low.
ONLY type C (Bixenon Hi/low bulbs) will need cutting of the back of the headlight housing to accommodate the increased depth of the bulb, all other H4 types do not require housing cutting.
1- The bulb with the high Halogen is unacceptable & only a fool would buy such a HID kit.
2- The single intensity H4 requires no headlight cutting but you loose high beam, you can always aim the the headlight higher to make it reach farther.
3- The Bixenon Hi/low bulb retains both hi & low beams but the bulb extends beyond the headlight housing requiring cutting the back of the housing to be able to aim the headlight.
4- the Dual Bixenon H4 is the newest H4 bulb but uses 2 ballasts & since its two bulbs it takes a good 50 seconds to full brightness when switching from hi to low or back which lets you drive in the dark for 50 seconds until it gets to its full brightness, this type does not require cutting of the housing but as you can see it has its drawbacks.
I posted two photos one of the Bixenon Hi/low which require cutting the back of the headlight housing & one single intensity that does not require cutting the back of the headlight housing.