The Second Assault DLC for Battlefield 4 was the first released expansion but only if we talk about the Xbox version PC gamer did start with Battlefield 4 and then China Rising but many PC players do wait already for the Second Assault DLC.
Now german premium users of the Battlefield 4 PC version saw an entry for the Second Assault DLC with a mentioned release date in the Origin client. Now it seems that the release date for Second Assault is set to February 18th. But it must also be mentioned that this date is not official confirmed by EA and DICE unless you mean that the Origin client entry is an official statement. But until today, nobody of the EA or DICE team have mentioned a release date for the PC version of the Second Assault DLC. But February 18th might be still true if we believe the entry in the Origin client.
Update: The release date for Second Assault is officialy revealed now.