4mm hook-up wire typically refers to electrical wire with a 4mm diameter. This wire is commonly used for various electrical and electronic applications, such as connecting components on a circuit board, making jumper wires, or wiring up small electronic devices.

It's important to note that there are different types of hook-up wire available, including stranded and solid core wire, as well as various insulation materials like PVC, silicone, or Teflon. The choice of wire type and insulation material depends on the specific requirements of your project or application, such as flexibility, temperature resistance, and voltage rating.

When working with electrical wire, make sure to follow safety guidelines and choose the appropriate wire for your needs, considering factors such as current carrying capacity and environmental conditions.