When I tried to update my iphone through both mobile or PC, kept saying an unknown error occured (4000)

There is a bug on iTunes for PC causing the error 4000. Try updating Wirelessly using WiFi.

A workaround has been reported for iTunes; that is to temporarily remove your iPhone passcode, update the phone through iTunes, and restoring the passcode (I cannot attest to this myself) but updating Wirelessly works and it is easier.

Update your iPhone, iPad, or iPod touch - Apple Support

Did you try through WiFi or using Mobile Data? Mobile data will not work, at least not on an iPhone 8.