Roy Tanck has released a web service '4K Desktop wallpaper generator ' that can randomly generate 4K resolution wallpapers for free, so I will actually use it to generate wallpapers for 4K monitors.

DesktopHut lets you set animated live video wallpaper as your Windows 11/10 desktop background. It is backed by a gallery of thousands of live wallpapers.

Lively Wallpaper is a free software design application that can transform your static Windows 11/10 desktop to make it look energetic and active.