September 18th, 2015: For information on what's new in 1.4 RC 1, see the release announcement. For additional information about Android Studio, see the main developer site.
The release is available in the canary channel, so you can switch to that channel (if necessary) and check for updates via Help > Check for Update... (on OSX, look in the Android Studio menu). This will download and install a patch rather than download a full IDE image. Note however that if you do this, you will replace your existing 1.3 installation, which you may not want to do. Just keep 1.3 and 1.4 RC 1 installed side by side; they already use separate settings and cache directories.
Windows: https://dl.google.com/dl/android/studio/ide-zips/188.8.131.52/android-studio-ide-141.2262011-windows.zip (353 MB)
Mac: https://dl.google.com/dl/android/studio/ide-zips/184.108.40.206/android-studio-ide-141.2262011-mac.zip (354 MB)
Linux: https://dl.google.com/dl/android/studio/ide-zips/220.127.116.11/android-studio-ide-141.2262011-linux.zip (360 MB)