In day four of the "12 Days of Retool" series, the focus is on demonstrating the use of Retool to perform basic CRUD operations (create, read, update, delete) using HTTP verbs for a REST API. The post explains how to create a resource for an API endpoint using Retool's Generate API from CSV tool and walks through testing these operations using a Books API. It outlines the process of sending GET, POST, PUT, DELETE, and PATCH requests to manipulate book records, emphasizing the ease of testing API functionality within a Retool app. The tutorial is part of a broader series aimed at showcasing Retool's capabilities, with previous posts covering topics like JavaScript operators, and upcoming ones set to explore additional features.