How to Add a Custom Tab in the Customer Account Page in Magento 2 (Without the 404)
A custom My Account tab needs a route, a controller, and a sidebar link working together — wire up only one and you get a 404. This guide builds all three on Magento 2.4.4 — 2.4.9 (Luma + Hyvä) and explains exactly why the 404 happens.