https://www.cnet.com/news/pixel-4-and-4-xl-rumors-price-specs-leaks-features-everything-else/

The Pixel 4 may have a feature that would allow a user to switch between two functioning phone lines. This is an update from the previous Pixel 2 and Pixel 3 (and their larger counterparts the Pixel 2 XL and 3 XL). Those phones have a standard SIM card slot and an eSIM. While you could have two numbers provisioned for the device, you could only talk, text or use data on one line, while the other remained inactive. In other words, only one SIM could be activated at a time.

But the Pixel 4 would be able to have two active SIMs. However, if either is being used to make a call or text, the other line won't be able to take calls or text for the time being. This is similar to how the new iPhones and other Android phones operate.