Ultima IV license
The license of the xu4 code as well as the Debian conversion is the GPL version 2.
The license of the graphics update is described as Freeware.
The license of Ultima IV is unclear. It is freely downloadable, for example from GOG; the end user license offered there says, among other things, that you are not allowed to distrbute it. Whether that still really applies is unknown; fact is, that it is being distributed at other places and that there has been no action taken against that by the copyright holders.