Lots of cross-buy and of course "indie" games in
May's PS Plus free offerings, giving us 5 PS4 games in total!
Guacamelee (PS4)
Ether One (PS4)
The Unfinished Swan (PS4/PS3/Vita)
Race the Sun (PS4/PS3/Vita)
Hohokum (PS4/PS3/Vita)
Murasaki Baby (Vita)
I own The Unfinished Swan, and it's definitely worth playing. I knew Murasaki Baby would come to Plus, so I'm glad I never bought it. Ether One looks interesting. I'm always game for an adventure/puzzle game. The others I'll pass. Guacamelee just isn't my kind of game. I played it for a couple of hours during the PS Now Beta and got a sense of why it got so many favorable reviews. I just hate beat em ups and executing combos -- I'm just not any good at it to be honest.
EDIT: Watching the video, I think I'll probably give Hohokum a shot too.