I think I am the one who introduced the nickname "core four", to refer to the four lineages that the Players Handbook features as "common". Heh, mainly I like the rhyme, "core four".
I similarly refer to the Wizard, Fighter, Rogue, and Cleric as the "core four" classes.
Both foursomes relate to old school D&D. They are the "traditional" four and retain their high status and priority.
Possibly, we are witnessing the first time that their popularity is slipping, relative to other lineages and classes.
When I say "core four", I mean the traditional four that are in core. (I dont mean, that core only has four.)