I can't update my iPhone 8 on my computer anymore. "An unknown error occurred (4000).

Did you try to connect in recovery mode?

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

Also check this discussion:

I got Error 4000 when updating iPhone - Apple Community

Hopefully, recovery mode will work for you, but that particular error code has been associated with a bug in iTunes for Windows. Apple should be well aware of it by now, but they haven’t fixed it.

Some other suggestions are....

• Temporarily disable your phone’s passcode before updating. When you do this, you lose all cards in Apple wallet and they will have to be re-entered.
• Downgrade iTunes to version 12.8 on your PC. This may a bit more work that you want to go through.
• Update over the air by going to Settings/General/Software Update