Some marketplaces add a fourth transacting party to solve a monetization problem that the standard structure can't fix on its own. Instacart's core marketplace is three-sided (consumers, shoppers, retailers); it added a fourth side — CPG brand advertisers — specifically because the transaction fee taken from the other three sides was capped and couldn't be raised without hurting the marketplace itself.
Casey frames this as "hard mode," not a default move: only reach for a fourth side when a specific, otherwise-unsolvable unit-economics problem calls for it, not as a general growth tactic.
Enabled the monetization mechanism described in Digital Slotting Fees (Instacart Ads).