I read and write through a translator, so I did not understand the order of your actions. I checked:
1. Clicked on the pdf file F3.
2. Pressed the number '4' on the keyboard and chose another plugin to view.
3. Closed browsing by Esc.
4. Entered cm_UnloadPlugins on the command line. Nothing happened.
5. Pressed F3 again and disabled plugins through the menu.
Again nothing happened.
What did I do wrong to reproduce your error?
And what version of SumatraPDF do you have?