According to a snapshot of Origin Germany, the PC release of the Second Assault DLC will be out February 18th for Battlefield 4 Premium subscribers. This would presumably be for other remaining platforms.
The notice is said to have been removed but a recent patches have been preparing for the DLC. Second Assault takes four BF3 maps and upgrades them for Battlefield 4's new engine.
This does fit the general timeline that has been floating around for Second Assault, which was a late February release. Non-Premium users would get it early March.
Remakes of Operation Métro, Operation Firestorm, Caspian, and Gulf of Oman are all exclusive to Xbox One for now.