This was released as free DLC.
Someone taking aim and firing a shotgun at infected zombie-like people in a shanty-town village. UI elements show things like the current score, points popping up from enemy hits, time remaining, ammo count, and combat streak.

Capcom released the first piece of post-launch DLC for their Resident Evil 4 remake early this morning. That content is none other than the fan-favorite Mercenaries mode. This mode blends together the combat encounters of the base game in battle arenas where you are tasked with surviving tons of base enemies, mini-boss spawns, the environment, and the clock as you work to achieve the highest score possible.

This release of the Mercenaries is listed as DLC and may need to be manually added to your Steam account. This initial launch of Mercenaries includes three arenas and four characters (Leon, Luis, Krauser, and Hunk) with different loadouts to choose from. It seems as though there is at least one location and two characters (Ada and Wesker) that may be coming later on, but I don't believe Capcom confirmed anywhere about the the amount of content Mercenaries would have prior to release. It is possible this extra Mercenaries content will drop alongside the release of the Separate Ways expansion where you play as Ada, but this has also not yet been confirmed nor announced.

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Race against the clock and battle your way to the top of the leaderboards!

First featured in the original Resident Evil 4, The Mercenaries is back! Are you ready to test your mettle in this thrilling, breakneck fight for survival against an increasingly dangerous horde of enemies?

Characters, old and new, can now use Mayhem Mode—unique abilities that add new layers of strategic and tactical depth never seen before in The Mercenaries!

Like the main story, The Mercenaries is reimagined and awaiting those who enjoy fast-paced action combat!​
Last night, Capcom also released a series of microtransactions for Resident Evil 4. Each one grants you access to different amounts of Exclusive Upgrade Tickets. These tickets allow you to upgrade a weapon's exclusive ability earlier than you normally would just by playing the game. They're optional purchases and for a lot of people aren't really necessary. They are even less necessary on PC, if you catch my drift. Still, if you want a shortcut and have the money to blow, then have at it. Live your life.