The categorization is a simplification made by designers at some point in D&D history to streamline designing and balancing the game.In D&D 5E the three pillars are combat, exploration, and social interaction.
Can we include shopping scenes in this one?The fourth pillar is Downtime.
This pillar includes domain work (i.e. home/base/castle/stronghold construction and management), treasury division, training, spell research, potion/scroll/item construction, family dealings, business dealings, investments, and all the other stuff not done in the adventuring field; each at either the party or individual PC level.
I’m pretty sure the Icewind Dale and Dragonlance books both include 5e rules for fishing mini-games.The fourth pillar of play should be a fishing mini-game.
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(I'm only half-kidding. I'd really like to see a supplement that has a bunch of mini-games in it for all sorts of situations: fishing contests, drinking contests, escaping pursuit, carnival games, etc. I know that these would fall under the "Social" pillar, but if done right, they'd be a lot more fun than the typical series of Charisma checks.)
Mass combat is something of a cross between downtime and combat though. Not sure where I'd put it. Probably downtime.The fourth pillar is Downtime.
This pillar includes domain work (i.e. home/base/castle/stronghold construction and management), treasury division, training, spell research, potion/scroll/item construction, family dealings, business dealings, investments, and all the other stuff not done in the adventuring field; each at either the party or individual PC level.
Like a shop keeper in Nethack!In my homebrew campaign, the party's hometown (and "home base" for their adventures) is a retirement community of old adventurers. All of the shop keepers, temple clergy, and government officials are high-level NPCs who all smirk and roll their eyes when these adorable little characters start trying to threaten or talk down to them.
I roll everything out in the open (we play on Roll20) so the players got a glimpse of the bartender's stats when he broke up a bar fight. "Plus ten?! The friggin' old bartender is swinging at plus ten to hit?!" They decided to behave themselves for the rest of the night, and every night since.
Architecture suggests that four pillars are perfectly stable.Three is the right number of pillars. You might change what those pillars are, but fewer makes the whole unstable, more makes it overcomplectated.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.