The most important change is for users of 5.6b3, the previous beta release. That version would hang for 10 seconds or more (sometimes a lot more) after you resized a file dialog. That’s fixed in b4.
Other changes include a fix for instances where a top-level folder in Google Drive would get its “hidden” flag set by a bug (or feature?) in Google Drive, resulting in Default Folder X hiding folders or files within that folder in the Recent Folders and Recent Files history. Now Default Folder X just ignores that hidden flag. And I’ve added support for the Mac App Store version of PDF Studio Pro, an app DFX was refusing to enhance because it’s written in Java, and Java apps can sometimes crash because they don’t properly support the macOS Accessibility API.
As always, details and download links are on the Default Folder X Testing page. Note that the change history there includes a list of all the changes made since the last full release (version 5.5.9), if you haven’t been keeping up with previous beta releases.