I am running IPD lowering springs and I wanted some input on the b6 (yellow and blue) Bilstein versus the b4 (black) Bilstein shocks. Basically, I don't want to spend way more than necessary on similar products but what would be the downside of getting the cheaper b4 shocks.

 

B4s are OEM replacements and do not have the same damping control as B6 which are stock length sport shocks. The B8s are the shorter length sport shocks designed for lowering springs. So net is, B4s are going to be like using stock struts - in fact the Boge/Sach shocks may actually be firmer and a better choice than the B4s.

 

i am running the ipd springs and they don’t offer b8s so is it fair to assume that the b6 are what’s best for their springs and won’t perform worse being stock length compared to the shorter ones?

 

yes If you have gone with a shorter / stiffer spring you'd want the B6s extra damping control to match better to the spring rate.