I forgot my password. How can I reset NETIO 4C to factory settings?

The proper procedure is described in the manual – look for “Forgotten password. Restore Factory Defaults”:

If you forget your password, it is possible to manually reset the device to factory defaults. This is done by pressing and holding outlet buttons 1 and 2 when starting the device via power switch.  Hold the buttons until the device beeps 3 times. During the resetting process, all the outlet LED are blinking red.  As soon as reset is completed, the device beeps 3 times. Whole procedure takes about 2 minutes.

For devices produced before 2016 (KOUKAAM company)

Factory default is done by pressing and holding outlet buttons 1 and 2 when starting the device via power switch.  Hold the buttons until the device beeps 2 times. During the resetting process, all the outlet LED are blinking red.  As soon as reset is completed, the device beeps 2 times.