Mobile is pushing aging web architectures to the brink. The three-tier architecture built for a browser-led PC world can't flex, scale, or respond to the needs of a good mobile experience or the emerging requirements for connected products. Mobile's volatility and velocity of change require a distributed four-tier architecture that we call an "engagement platform." The engagement platform separates technical capabilities into four parts: client, delivery, aggregation, and services. App developers need to start planning immediately for the migration from three tiers to four. This is an update of a previously published report; Forrester reviews and updates it periodically for continued relevance and accuracy.