Hashim Warren introduces Challenge 4 of the #100DaysOfGatsby, encouraging participants to integrate third-party React components into their Gatsby sites by adding a form using the Formik library. Gatsby, based on the React framework, allows developers to leverage a vast ecosystem of plugins and themes, as well as thousands of React projects. The challenge suggests incorporating the form into an "About" page previously created and provides resources for importing and using React components in a Gatsby project, specifically recommending Formik for crafting a newsletter signup form. Participants are urged to adhere to accessibility standards for forms and are offered support through the Gatsby and Formik communities via Twitter, Discord, Reddit, and Dev platforms.