Web application architecture defines the interactions between applications, middleware systems and databases to ensure multiple applications can work together. According to this article some big changes to web application architecture since 3-tier include
Service-oriented architecture, in which a lot of the code for the full application exists as services, each with its own http API. So one part of the code can make a request to another part of the code, often running on a different web server. The API is frequently organized according to REST design principles. The request and the response are typically just data, not necessarily human-friendly presentation.
Single-page application, in which web UI is provided by a rich Javascript application, which stays in the user's browser over many user interactions. The UI can perform many asynchronous or synchronous requests to the web server without reloading the page. It uses AJAX or WebSockets for this communication. This gives the user a smoother more natural experience, without being interrupted by page loads.
Together these two trends have enabled web apps to be designed for multiple platforms. This is important in the modern world when many users are on tablets or phone devices, instead of full-screen browsers on laptops or desktop computers. The user interface can be designed to fit smaller screens, even while most of the code for the app remains the same.