You can uninstall the previous version but your plugins and presets will also be removed , you have to back them up youself.
Am I required to remove CS4 version if I have Adobe Creative Cloud and download CS6? If I "turn-off" cloud will the CS4 still be there, or does it get written over or disabled?
You are not required to remove CS4. Both CS4 and CS6 can co-exist on the same machine. If you quit your paid membership (or your free trial expires) then CS4 will still be there and work.
A Creative Cloud membership uses your Adobe ID to license the product. CS4 uses a serial number to license the product.