The perpetual 4K B plug-in license is available for the original owner only. It is a non-transferrable license.
It is not possible for SSL to transfer ownership of the 4K B license if a unit is sold second-hand. As a second hand user, you can subscribe to the SSL Complete bundle to access the 4K B plug-in.
If you have purchased a UC1 second-hand you will need to contact the seller regarding the transfer of Channel Strip 2 and Bus Compressor licenses to your iLok ID. For instructions on iLok license transfers see https://help.ilok.com/faq_licenses.html