macOS 11.4:
The Nik4 versions of Viveza and Silver Efex Pro will not run correctly as plugins in Affinity Photo nor PhotoLine. When running in either of these hosts, the plugin shows no “OK” button to return results to the host. File->Save produces a “processing” message on the screen that hangs. The other plugins in the suite run correctly as did the Nik3 collection.
No problem running as standalones or launching from PhotoLab 4.
(I have filed a trouble report with support.)