Which respository does xvnc4viewer belong to?

The xvnc4viewer package was removed from the Ubuntu repositories after Ubuntu 18.04.

$ rmadison xvnc4viewer
 xvnc4viewer | 4.1.1+xorg4.3.0-37ubuntu5     | trusty/universe          | amd64, armhf, i386, powerpc
 xvnc4viewer | 4.1.1+xorg4.3.0-37ubuntu5.0.2 | trusty-security/universe | amd64, arm64, armhf, i386, powerpc
 xvnc4viewer | 4.1.1+xorg4.3.0-37ubuntu5.0.2 | trusty-updates/universe  | amd64, arm64, armhf, i386, powerpc
 xvnc4viewer | 4.1.1+xorg4.3.0-37.3ubuntu2   | xenial/universe          | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 xvnc4viewer | 4.1.1+xorg4.3.0-37.3ubuntu2   | bionic/universe          | amd64, arm64, armhf, i386, ppc64el, s390x

• The rmadison command is included with the devscripts package.
• You can get the same information in a different format by browsing http://packages.ubuntu.com

Note that the package is in the -universe pocket, and is still available for Ubuntu 18.04 (but not for newer releases). Universe packages are not part of a stock install of Ubuntu. They are still Ubuntu packages, and we still support them. They're just not included with the Ubuntu installer.

Also, keep in mind that doing a release-upgrade (from, say, 18.04 to 20.04) does NOT necessarily remove the older software. If a 20.04 package exists, it replaces the 18.04 package. If a 20.04 package does not exist, then then 18.04 package remains installed on the 20.04 system as long as it is compatible.

xvnc4viewer isn't part of the default repositories for Ubuntu 20.04 (Focal Fossa), but it is part of the default repositories for Ubuntu 18.04 (Bionic Beaver). The exact version of the package, 4.1.1+xorg4.3.0-37.3ubuntu2, also matches that. So I'd assume that the system in question has previously been running 18.04 and has been upgraded since.

apt policy only mentions repositories that currently are in the system's source list. So the system might previously have the repositories for Bionic set up, xvnc4viewer was installed from there, and later the Bionic repos were replaced with the Focal ones during an upgrade. In this case, the repository the package originally came from isn't in the source list any more, and apt policy can't list it.

The other possibility, as Artur Meinild already mentioned, is that the package was installed manually with dpkg, with apt never coming into play for this package.