EA has made all five of Battlefield 4’s expansion packs completely free until Sept. 19, across all platforms. This means you can snag them for no charge whatsoever, and keep them forever—if you do so by Monday.
The actual base game of Battlefield 4 still needs to be purchased separately, but on PC at least, you can grab the expansion packs without owning the game in your library. This essentially means that you can grab the expansion packs now and buy the game later.
The expansion packs available are China Rising, Second Assault, Naval Strike, Dragon’s Teeth and Final Stand. Each of these have a community mission going on, although the first has ended. The community mission for Second Assault is currently active, and will end on Sept. 18. It will be followed by the Naval Strike mission, ending on Sept. 20; then the Dragon’s Teeth mission, ending on Sept 22; and finally, the Final Stand mission, ending on Sept. 24.
Battlefield 4 and its expansion packs are available on PC (via Origin), PS3, PS4, Xbox 360 and Xbox One. On both Xbox consoles, you will require an Xbox Live Gold account. For details on the community missions and how to grab the expansion packs, check out the Road to Battlefield 1 page.