Recently the familiar projection library PROJ incremented the major version number from 4 to 5, as described in the NEWS file. Version 4 of this software is popularly known as PROJ.4, however newer releases will use it's original name, PROJ.
(The name of the organizing project remains as proj.4, I think mostly because http://proj.org is not available).
The issue on GIS.se is that the current tag is proj.4. I suggest renaming it to proj to be version independent (keeping proj.4 as a synonym). Also, to be inline with Stack Overflow's
proj tag, which is equivalent.