This is a pretty old finding, but I didn't see a tutorial for it and I thought I'd type one up. This is good if you damage your PS4 somehow when trying to mod it and it can't boot.

Tutorial:
Turn off your PS4
When it is fully off, press your finger on the power button and you should hear a beep
Keep your finger on the power button for 7 seconds until you hear another beep
Release the power button and your PS4 will boot into safe mode
Connect your controller via USB cable as wireless isn't available

Options available while in safe mode:
Restart System - Ends Safe Mode and restarts the PlayStation 4 normally.
Change Resolution - Changes the screen resolution to 480p when the PlayStation 4 is restarted.
Update System Software - Allows the PlayStation 4 to update the system software via Internet, USB Drive, or Disc.
Restore Default Settings - Restores the PlayStation 4 to the default factory settings.
Rebuild Database - Scans the drive and creates a new database of all content.
This operation may take a long time depending on the type and number of data items.
Initialize PS4 - IMPORTANT: All data and settings will be lost by performing this step.