When using nmap, the nmap prompts an error:    Failed to open device eth4.

After investigation, WinPcap caused it

solution 1:

Uninstall WinPcap and reinstall WinPcap

WinPcap download address:   http://www.winpcap.org/install/default.htm

solution 2:

Uninstall nmap and winpcap and reinstall nmap

you can use nmap normally by following one of the above methods