What year was 4 digit phone numbers used?

Telephone numbers started as one or two digits. Then, they had to add an exchange as local subscribers increased.

In the USA, they used a name exchange+ personal number, with the name being an area in a town/city. The name-exchange was an easy way for customers to recall the exchange. For example, Able-5555. Locally, the name was further abbreviated, such as AB-5555 for Able-5555. When making a call when only operators connected the call, you'd just give the local name (exchange) of Able with the personal number (ex. 5555). Later, you could dial AB-5555.

By the 1960s, rural towns/cities went to a number as the exchange in place of the name. So, 555-5555. Larger towns/cities might have converted earlier.

The 7-digit numbers stayed until about the 1990s when telephone companies required area code+exchange+personal number.