The Pixel 2 was the first of Google's devices to support the eSIM standard, which allows you to switch carriers without inserting a physical SIM card. Now that the Pixel 4 is sold on all major carriers, more networks can be activated using eSIM — including T-Mobile.
According to a T-Mobile support page, the December update for the Pixel 4 included more than just the usual security fixes — support for activating a Pixel 4 on T-Mobile via eSIM is now included. While the main advantage with eSIM is that you don't have to go to a store or wait for a mail delivery to activate your phone service, this also means you can have another network in the physical SIM slot and receive calls/texts from both.
This isn't entirely a surprise, as placeholder support documents for eSIM support were published months ago. Some customers were even able to manually activate eSIM on T-Mobile by contacting support. Now the process should be much easier.