Jamie, tbh when they quote like -35mm etc they dont actually go down that much precisely! seems stupid but i think theyre more of a guide! i lowered mine by "-40mm" and to be honest it didnt drop by 40mm!!there is still loads of gap at the front between the tyre and arch. it dropped a noticeable amount but no way 4cm! with a 20mm drop i dont really think that youll actually see the difference! go for 35mm drop mate, u wont be dissapointed!