scrcpy 4.0 arrives with major upgrades, including a migration from SDL2 to SDL3 for improved stability and new capabilities like aspect-ratio locking during window resizing. The update introduces flex display support, enabling dynamic window sizing synced with client applications, demonstrated via commands like `scrcpy --flex-display -x --start-app=org.mozilla.firefox`. Users can now control camera torch and zoom via keyboard shortcuts (MOD+t/MOD+Shift+t for torch, MOD+↑/MOD+↓ for zoom), with startup torch activation via `--camera-torch`.

A dark gray default background replaces black, customizable via `--background-color`.