Although all of the hype is one Battlefield 1 right now, DICE has recently started to release some of their previous DLC for Battlefield 4 as free content. Plenty of DLC packs are now readily available to 'purchase' for free from the Sony, Xbox and PC Marketplaces from any web browser.
The latest Battlefield 4 DLC pack to go free is the Navy Strike DLC. This pack includes a total of four new multiplayer maps, five weapons unseen by players that do not own the DLC pack, as well as two gadgets and a new hovercraft vehicle that can be used on water and land. As you can imagine, there is quite a lot of water on the Navy Strike maps.
The Carrier Assault mode is also exclusive to Navy Strike DLC owners, but now that it is available to download for free, everybody will be able to play the game mode. Usually the Navy Strike DLC cost $15 so if you'd like to gain access to it whilst it's free, you should sign into the PSN marketplace, the Xbox Games Store or the Origin PC app to claim the content as soon as possible.
Previous DLC released by DICE as part of the 'Road to Battlefield 1' campaign was only made available for free for a limited period of time. It's likely that this will apply for the Navy Strike DLC too. Once you have claimed it, however, you will be able to use the DLC for free forever.