Is Gamedonia compatible with Unity 4.x?

Yes, it is!
You may find, though, that when importing the Gamedonia SDK into a Unity 4.x project, the Gamedonia prefab looks like a blank file. Here is how to solve the issue:

You just need to create a new Gamedonia object in the editor. Game Object > Create Empty. Name it "Gamedonia" and drag the GamedoniaBackend script to it. The GamedoniaBackend script is located at Assets/Plugins/Gamedonia. Also check that if you go to Edit > Project Settings > Script Execution Order, the order of the script is set to -3 (this should be its default value). Then set the Api Server Url to http://api.gamedonia.com and you're good to go.