My understanding is that the only diff between the two is that the 5100 front shock is height-adjustable (and they're different colors - 4600 is blue/yellow while 5100 is silver/black). Others may know more details (or contradict me, I'm no expert).
I just got off the phone with Bilstein and edited post #47 accordingly. The 4600 and 5100 series DO ride differently. See post #47 and #48