Here are the changes specifically to Ubuntu Cinnamon for the release, only containing the main important parts, and excluding the bug fixes behind the scenes:

Cinnamon 6.0.4

Desktop

• The sound applet now rounds labels for the artist song names
• The menu editor window can now be resized
• Desktop backgrounds can now accept the AVIF file format
• Ability to download Nemo Actions with other spices
• Experimental support for Wayland
• Minor fixes

Screensaver

• Note: The screensaver is disabled in Wayland sessions
• Stability fixes

Nemo

• Various fixes (style, memory leaks)

Muffin Window Manager

• Now has experimental support for Wayland

Live Image

• Now uses the Flutter-based installer!

For more information about Ubuntu Cinnamon under the hood, see the official Ubuntu release notes for more detailed technical information.