Just saw this at gHacks.net, with a good review by Martin Brinkmann.

There are some major changes. First, it’s supported on W7 SP1 and newer Windows OSs. So, if you have anything from before them, stick with the older version. The new one requires .NET Framework 4.5. It will be installed automatically when installing Paint.net 4.0 if not on the target machine already.

“Paint.net 4.0 ships with a new rendering engine that supports multi-core processors and GPU hardware acceleration among other things. As far as performance is concerned, you may notice huge improvements in this area. Startup performance is improved thanks to the Multicore JIT feature of the .NET Framework 4.5. You may also notice a drop in memory usage, faster rendering thanks to the new asynchronous multi-threaded rendering engine and responsiveness improvements especially when working with large images.”- gHacks

There are improved selections, and hardware acceleration (in Settings). ‘Settings’ has been improved as well as resizing and Red Eye Removal. According to the review, many of the tools have been improved and the program is listed under default programs making file handling easier.

Please take a look at his review here: http://www.ghacks.net/2014/06/25/image-editor-paint-net-4-0-final-released/?_m=3n%2e0038%2e1286%2ehj0ao01hy5%2e1c0o

Enjoy!