FxPlug 4 - How can know if I am rendering to final render view or a thumbnail?

This information is not available to plug-ins. What you do get is the pixel transform of your inputs and outputs. They should allow you to properly scale your parameters and drawing. For more information on how that works, please see this documentation. In general a plug-in should not be concerned with how they are being asked to render, they should simply follow the parameters the application supplies. Otherwise it creates a confusing workflow for the users.