The Cobra Mk IV is available to purchase using in-game credits for players who purchased both Elite Dangerous + Elite Dangerous: Horizons before a certain date (dependent on platform).
PC users are eligible to purchase the Cobra MKIV with in-game credits if they purchased Elite Dangerous + Elite Dangerous: Horizons before 5th Feb 2016. This includes Lifetime Expansion pass holders, Alpha & Premium Beta owners.
Xbox One users are eligible to purchase the Cobra MKIV with in-game credits if they purchased Elite Dangerous + the Horizons Season Pass before 30th July 2016.
The Cobra Mk IV has not been made available via PS4.