Both K4s are the same speed. It's just the contrast, background are better than most Silvers out there. The page and power buttons also have been tweaked a bit.
How are the page buttons different? I have a spinal cord injury that makes my hands very weak. The K3 keyboard's buttons were easy for me, but the weight makes it difficult to hold. The K4 NT silver was easier to hold, but the buttons were smaller and hard for me to push. I'm now back to the keyboard, but if the K4B's buttons are easier to push - I may consider getting one of those.