Essentially, the 4600 is Bilstein's OEM replacement shock for stock rigs, and the 5100 is the aftermarket shock for modified rigs.
What the kit makers do (AC, etc) is sometimes find a larger truck that has a STOCK shock that can work as a lift kit shock on a SMALLER truck...so they can save a few bucks by using an OEM shock AS a lift kit shock.
That's the same thing they did with the HD series and 5100 shocks...found a larger truck's stock shock (HD) and use that instead of the 5100.
The key is to find a larger stock truck that uses a shock with the right valving, compressed and extended lengths...to work as the lift kit shock on the smaller truck.
After that, its about how long it holds up under off road abuse, etc. Internally, the HD and 5100 were close enough that most people were fine with either version. I have yet to MEASURE a 4600, but, I suspect that it too will be like the 5100 internally.
The exteriors are different, paint vs treated metal, etc, and eye/stud options...but if the valving and lengths work, its probably OK. Until I measure a set though, I don't know that for a fact.