Since you have the equipment for both, read
Wired Bridge with two WAPs using the same SSID to see how to run two routers on the same connection
properly. It's a step-by-step hand-holding "manual". (But be aware - routers [hubs - same thing] are designed to transmit and receive horizontally - going 4 floors may require 4 routers arranged so that they're
not sitting one on top of the other. [The path loss for two devices with horizontal antennas sitting one above the other, about 8 feet apart, can be enormous. That's why you'll see towers with 2 two-way radio antennas one above the other. Just the placement keeps a lot of the transmitted signal out of the receiving antenna.])