Can 4 thhn be usedto feed 100 amp subpanel?

Yes, 4 THHN (Thermoplastic High Heat-resistant Nylon-coated) wire can be used to feed a 100 amp subpanel, as it is typically rated for 85 amps in a residential application when considering 75°C termination. However, local codes and conditions, such as the length of the run and the type of insulation, may affect the final decision. Always consult with a licensed electrician to ensure compliance with the National Electrical Code (NEC) and local regulations.