SNK announced during their PAX East panel that Metal Slug XX, which originally released on PSP, will be coming to PlayStation 4 in Summer 2018. [Thanks, Dualshockers!]
The game will be available digitally through the PlayStation Store, and more information will be released later on the new features.
Metal Slug XX was originally a port of Metal Slug 7 that added new alternate routes and ad-hoc multiplayer support to Metal Slug 7 on Nintendo DS. Furthermore, while Ralf and Clark were originally available as playable characters in Metal Slug 7, Metal Slug XX also added Leona Heidern from The King of Fighters as a DLC character.
Apart from the PSP release, it was also available on Xbox 360 via the Xbox Live Arcade.
Metal Slug XX is available for PlayStation Portable and Xbox 360, but will be released for PlayStation 4 in Summer 2018.