As far as 4600 vs 5100 goes, they aren't the same. From
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27023684/srt/pa/pging/1/page/1.cfmStars101 said
"Bilstein tech support said the 5100's are velocity valved for off road applications. They are not recommended for TC or heavy towing/hauling. These should have 4600's which are linear valved shocks."
"DH said Bilstein Tech Support (out of Moorseville NC) was terrific. Brian spoke to him for nearly 20min. about the various applications.
The way I understand it is the 5100's are great for lifted or off road trucks that will get a sharp movement, i.e. big pothole. They dampen that type of quick jarring movement. But they don't do anything for the rolling/rocking motion you get with a top heavy truck camper. That's when you should choose the 4600's.
If you are a gear head give them a call b/c the actual interior design/technology of the shocks is different between the two series. And I am not smart enough to get what that is exactly... All I know is that on a stock '06 2500 Dodge Ram CDT 5100's were dangerous with a 1,500lb. TC. DH wouldn't even let me drive it back to the dealer with the TC loaded (25 miles) for the replacement. He's doing it himself at home after work."