From what I've seen online when they just list them as Heavy Duty they're referring to the B6, which are also blue and yellow. But if I'm going to spend the money on new shocks I want the ones that will work the best for my purpose. So guess I'll stick with the 4600's.
Thanks for the replies.
P.S. Ok, Shockwarehouse.com says;
"The Bilstein Heavy Duty is a high pressure gas charged monotube performance shock.
Also called the Bilstein B6 Series or Bilstein 4600 Series, the Heavy Duty has an extruded metal body, high tech monotube construction and 360 psi gas pressure to prevent aeration and fading, it is the best performing and most durable unit we carry. No other shock can match it."
So I'm guessing that it depends on if the shocks are for a car/suv or truck might be the difference. But, according to them, if I order the Heavy Duty I'll get the right ones. Hope I'm right.