an unknown error occurred, how do i fix 4013

As a last resort, try a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are really only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation

Or, if the error continues.....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations