F4 recently began playing in my second monitor. How do I remedy this? I have not changed any settings so I don't understand why it would randomly begin to start in it.

Windows should save which monitor the application was previously opened on. Open Fallout 4, set it to Windowed Mode. Drag Fallout 4 to the desired monitor. Close Fallout 4. Open Fallout 4, set to fullscreen.

I don't have a multiple monitor setup so I cannot confirm this.

You are a beautiful individual. It is now starting in my primary monitor but now I gotta fix the black screen issue.