Well, that does seem to indicate the tab setting is set to 1, but only for that document.
You can start with a clean configuration by removing the .slickedit directory in your home directory - that just contains user configuration, so it can always be removed without having to un-install. Hopefully this isn't necessary.
So one clarification: when you go to Tools->Options->Languages->Application Languages->C/C++/Formatting and look at the indent settings there, those are the settings for the language. You open a C file for the first time, these are the settings that are used.
But some of the SlickEdit properties (like tab width and syntax indent) can be changed for a particular file. Adaptive formatting changes these per file settings, and these file specific changes are remembered, so the next time you open that file, the changed settings for that document will still be there, even if you've disabled Adaptive Formatting in the meantime.
The nuclear way to 'forget' these particular per-file settings is to exit SlickEdit, remove the ~/.slickedit/20.0.1/vrestore.slk, and then restart SlickEdit. In this case, you can probably go to Document -> Tabs, and set it to +4, and that would take care of it for that file.