Gamepointsnow who correctly reported last month's European PS Plus offerings correctly ahead of the official announcement have listed April's new games on Instant Game Collection as follows:
PS4: Mercenary Kings
PS3: PES 2014, Sly Cooper: Thieves in Time
Vita: Hotline Miami, Moto GP13
Mercenary Kings is no huge surprise as it follows other indie titles that were featured on the stage at Sony's E3 press conference as free PS Plus offerings upon release (Don't Starve, Outlast and Contrast preceeds it).
Interesting to see Konami offer up PES 2014 free of charge on PlayStation 3, perhaps in an effort to have some FIFA faithfuls give the game a chance ahead of next year.
We expect the official reveal by Sony soon. Stay tuned for confirmation.