I'd go 5u for a power supply, unless you've got room depthwise to put it on the back.
I'd go 5U as well since the Kemper has some rubber foots. Without removing them 4U will probably not provide enough space...
Yes, I'm using this combination, which works fine for me. But to be honest, i haven't tried something else so far...:-)
Hi , I have a 4HE, Kemper + Velocity 300, not problem yet 
Removing the feet is easy. So 4u are enough if you are fine with using two power cables or one y power cable. If you want to get some other stuff like a guitar wireless or a power strip with overload protection I would go with 5u.
You've already found the Thon Eco racks, so I assume you might be looking for a lightweight solution. The SKB XRack could fit your needs.
If you want a superior build quality and great looks, I recommend Tube Town Custom Racks ...of course those are quite pricey 
