You needn't remove the previous version of Poseidon. The older version will be replaced with the new one. Nevertheless, during Poseidon 4.0 installation, there is recommendation to make several backups.
I installed Deneb on my a4k through usb mouse and keyboard connected to Highway. So there isn't no need to remove the Highway (and probably Subway) too, if you plan temporarily use both usb controllers e.g. for testing reasons. I only noticed, that at the end of installation the usb stack was disabled, so I had to finish the installation with classic mouse. After reboot, everything worked fine.
Highway & Deneb in OS3.9
