Rejoice plugins developers, FlashDevelop 4 introduces project storage.

Storage isn't available in IProject, you'll have to cast it to Project:
- Add a reference to ProjectManager in your project
- cast PluginBase.CurrentProject (IProject) to Project

Now your project object has:
- project.Storage : Dictionary<string, string> where you can add new key-value pairs,
- project.Save() : persist your data (it will not be written automatically)

Thanks Philippe!