On https://wikibase-registry.wmflabs.org/wiki/Item:Q4 hover over "Concept URI" (left side bar).
The link starts with https: https://wikibase-registry.wmflabs.org/entity/Q4
However, this does not work in a query, while http concept URIs do work.
Example query (with URIs fully spelled out to not hide the details):
Wikidata concept URIs also start with http.