This template is called by Template:External icons. It makes a single icon with external link and sets the SMW property accordingly.
Currently it is mainly used by the parent template (but for the Wikipedia Icon, it is called from the object infobox templates) and always takes all 4 parameters.
Note that if the URL is just '#' then it does not display a link. Use this as a dummy empty value to prevent a value being set (e.g. by software).