The news comes from PlayStation Europe's official Twitter account, which also specifies that the beta will be available starting at 9 AM PST / 12 PM EST. The beta is scheduled to end on December 13, though it's unclear if that's still the case given that the start-date has been brought forward.
The Uncharted 4 multiplayer beta client is around 7 GB. To access the beta, you'll need to have purchased Uncharted: The Nathan Drake Collection. As of yet, Sony has not announced any other ways to get in.
According to DualShockers, the Uncharted 4 multiplayer beta comes with two maps and one mode. The beta represents just a slice of what will be offered in the full game. In addition, your progress in the beta will not carry forward.
Uncharted 4 launches in March 2016 exclusively for PS4. We may learn more about it soon, possibly during The Game Awards on Thursday or PlayStation Experience this coming weekend.