Version 4.0.20 introduces Dark Theme support, significantly improves High-DPI support, adds Explorer thumbnails for TGA and DDS image types, and fixes a lot of small bugs.
Version 4.0.19 fixes thumbnails in Explorer for non-.PDN images (e.g. PNG, JPEG) if Paint.NET was configured to open them by default. There's also a fix for a startup crash that some people were having if their Documents folder was redirected to a network share.
Version 4.0.18 is the first release that is available on the Windows Store! There have also been some significant optimizations for startup performance.
Plugins are supported! Instead of putting them in the installation directory, go to your Documents folder and create a new folder called "paint.net App Files" (without the quotes). Then, create the usual subfolders (Effects, FileTypes, and Shapes) and place the relevant plugins into each of them.