According to the National Electrical Code (NEC), the 4 AWG copper wire amp rating for  THHN/THWN-2 at 75 °C is 85 Amps. The same goes for copper RHW, XHHW and USE. 

The 4 AWG copper wire amp rating for XHHW-2, USE, RHW-2, and THHN/THWN-2 at 90 °C is 95 Amps. 

4 AWG aluminum wire amp rating of RHW, THHW, THWN, and USE at 75 °C is 65 Amps. 4 AWG amp rating of aluminum RHW-2, THHW, THWN-2, USE-2 at 90°C is 75 Amps. Finally, the 4 AWG wire amps of TW, UF at 60°C is 55 Amps. 

The 4 AWG wire amp rating depends on various factors, including the temperature, type of conductor (aluminum or copper), thickness of wire, etc.

What is 4 AWG wire used for?

4 AWG wire is of medium thickness and is used in situations that need a decent amount of electrical power flow. It’s good for situations where a balance between power transmission and flexibility is crucial. Let's take a closer look at where it's often used:

• Service entrances. The wire can handle around 85 to 100 amps of electrical current. This makes it suitable for bringing power from the outside electrical grid into homes and buildings. It's also used in other places where reliable power distribution is important, like powering appliances and equipment.
• Subpanels. 4 AWG wire is used to connect subpanels, facilitating efficient power distribution in various parts of a building. These subpanels, often located in areas like garages, workshops, or home offices, receive power from the main electrical panel and distribute it to designated zones. For instance, a subpanel in a garage might power tools, lighting, and charging stations for electric vehicles, while a subpanel in a workshop could energize heavy machinery and lighting circuits. 
• Welders. 4 AWG wire carries the power from the electrical source to the welder. It is used in many settings, such as shipyard welding, construction sites, automotive repair shops, pipeline welding, etc. 
• Large appliances. 4 AWG wire is used to connect large appliances like electric stoves, ovens, clothes dryers, and water heaters to the electrical system.
• HVAC systems. 4 AWG wire is used to connect components like air conditioning units and heating systems. 
• Backup generators. 4 AWG wire plays a crucial role in connecting the generator to the main electrical system of a building. It’s used  in refrigerators, sump pumps, heating systems, and lighting circuits during power outages.
• Battery banks. In renewable energy setups, like residential solar systems or off-grid cabins, 4 AWG wire connects batteries within battery banks. For instance, it links batteries to store excess solar energy during the day for nighttime use. Moreover, it also powers lighting and appliances in off-grid cabins, and supports uninterrupted energy supply in emergency backup systems. 

Common Types of 4 AWG Wires

There are lots of different types of 4 AWG wires. Let’s review some of them: 

• THHN/THWN. This wire has thermoplastic insulation and it's heat and moisture resistant. It's common among HVAC systems, lighting, emergency power systems, power distribution, etc. 
• XHHW/XHHW-2. This wire is common for powering lights, HVAC systems, motors, and industrial machinery. It's widely employed in pump stations, emergency systems, data centers, industrial control panels, etc. 
• SER/SEU. It is used for residential electrical services. SER/SEU supplies  power to apartments and commercial buildings, provides electricity to outdoor structures, and enables power connections in agricultural buildings. It's also used for temporary power setups in construction sites, subpanels, and mobile homes. 
• USE (Underground Service Entrance) Cable. This cable is used for residential electrical service and outdoor lighting. It also powers detached garages and workshops, supplies electricity to in-ground pools and spas, and wires garden sheds. 
• MTW. This wire is used for machine tools, motor connections, industrial automation systems, packaging machinery, and material handling equipment.
• Welding cables. Welding cable in size 4 AWG  is commonly used for different tasks, including welding, working on cars and big equipment, building things, and pipelines.
• Marine Cable. This wire is used on boats and ships. 4 AWG wire distributes power to navigation lights, HVAC systems, and charging systems. Its structure allows them to withstand harsh weather conditions and provide reliable electrical connection.