A new backup engine. We have redesigned the backup engine's architecture to make it more efficient and more robust.
More checks in ALICE and ANGIE to warn you of more common issues and let you work around them easily.
Interface improvements. We made some aesthetic and functional improvements in several places of the component to make your life easier and improve the way the component looks.
A new Amazon S3 integration. The new integration (Upload to Amazon S3) uses the Amazon PHP SDK for improved compatibility and performance. The old integration (Upload to Amazon S3 (legacy API)) which doesn't support all Amazon S3 regions will be removed in a future version of Akeeba Backup.
Deprecated features. Some features are now marked as deprecated and will be removed in a future version of Akeeba Backup: Site Transfer Wizard, Lite mode, System Restore Points, Extension filters. These features will be gradually removed after January 2015.
...plus several bug fixes and minor improvements.
[HIGH] Stack filters wouldn't load. As a result, voluminous and unwanted data of Joomla!'s Finder tables would always be included.
[HIGH] System Restore Points didn't really work
[HIGH] The Quick Icon - Akeeba Backup Notification plugin was broken since 4.1.0.rc1 (thanks Camden!)
[LOW] Upload to Dropbox may not work on servers without a global cacert.pem file
[LOW] Work around Joomla!'s bug causing it to not load its library translation strings while showing the installation status messages when System Restore Points are enabled
[MEDIUM] The backup ID was not returned to JSON API requests, making download after backup with Akeeba Remote CLI impossible (thanks Mikkel)
Brand new icon set in the Control Panel page by Helvecio da Silva (http://hlvcdesign.com.br)
Warning added when Joomla!'s com_postinstall component is broken (with instructions to fix it)
Less intrusive display of the file integrity check results