This is a quick and dirty plugin to replace the removed Reference Images Docker in Krita 4.0.0
As said at https://krita.org/en/item/krita-4-0-0-released/, the removal is only temporary:
"The Reference Images Docker has been removed. It was too easy to crash it if invalid image files where present. In Krita 4.1 it will be replaced by a new reference images tool."
Open Krita, in Settings -> Manage Resources click on the Open Resource Folder button.
Copy-paste all files from https://github.com/antoine-roux/krita-plugin-reference/archive/master.zip in the pykrita folder
Add execution permissions to the file reference.py (stored inside the 'reference' folder).
Restart Krita.
Enable the plugin in Settings -> Configure Krita -> Python Plugin Manager (Reference)
Check Settings -> Dockers -> Reference Docker
Click on Open and choose an image. Beware, no verification is done on the format... You've been warned!
You can grab the reference to move it (with any mouse button, left/right/middle... don't care).
Use the wheel or press on Ctrl and drag up-down to zoom.
Just click on the wanted part of the reference, you can shift your selection by some pixels.
A color preview is displayed on the top of the Docker.