If I read correctly, the RK005 is a USB MIDI Host/Hub and the RK004 is a MIDI router of sorts (merger/thru etc.). It sounds like you have all of the pieces that you need. Could you tell us more about how you’re setup, what you’re trying to achieve, and where you’re having issues?
Could be a cable routing or hardware config issue. I know the Keystep has dip switches on the back that read a bit counter-intuitively. What looks like a switch in one position is actually the opposite. They are set in different positions to use USB, DIN, or CV.
More infos please!
Edit: if you are looking for an actual USB hub that you can hang off of the RK005 so you can support more USB MIDI devices through it, I believe you want to find not only a powered hub but one with MTT (multiple transaction transistors). I bought a powered USB hub so I could connect multiple MIDI devices to my laptop and I find it is finicky at best. I’ve yet to look for another one as it’s not as important but I believe that would be the key to this scenario here.