Battlefield 4 Second Assault is available for all players to purchase and access beginning March 4th.
Battlefield 4 Premium Members, however, have the chance to jump in as soon as February 18th, thanks to the early-access perk included as a part of Battlefield 4 Premium.
PC Players simply need to jump into their Origin client, click on Battlefield 4, then the gray "i" information icon, select the Second Assault tile and click download.
Note, Premium players with early access will see Second Assault (Early Access).
Coinciding with Second Assault's release, there is also a patch for all Battlefield 4 players. Please be sure to download both the patch and the Second Assault downloadable content in order to play.
Console players can find Second Assault available in the Battlefield 4 in-game store during the following, respective time frames:
Xbox 360: 1:00AM PST -- 5:00AM PST
9:00AM GMT -- 13:00 GMT
PlayStation 3 and PlayStation 4 (North America): 2:00PM PST -- 6:00PM PST
PlayStation 3 and PlayStation 4 (Europe): 2:00PM GMT -- 6:00PM GMT
Note, Xbox One players should already have access to Second Assault.