In the fourth phase of the #100DaysOfGatsby coding challenge, participants are tasked with adding a Shopify ecommerce store to the fictional startup AudioC0RE's website, following their successful integration of a WordPress-powered blog. The challenge requires creating a swag store under a new "/shop" route, utilizing a Shopify Developer account and the gatsby-source-shopify-experimental plugin, introduced at a Gatsby conference. Participants are directed to develop a landing page and product pages for a shirt and a hat, with support available from the Gatsby community through forums and a coding livestream. The challenge aims to enhance participants' skills in integrating ecommerce solutions into web projects using Gatsby, a popular framework for building performant websites.