I did use PS4. Still it says, for PS3 only.

that's probably only the PS3 page for the game

"Forbidden Siren" with PS4 compatibility is listed in the german PS4 store aswell and that one shows up like an unowned game with a price

it says ps4 for me

I really don't know where you get this. Even in your picture, date of release says 2009, quite soon for PS4 game. And at least my store (Czech, but language is in english) in info, it says: Please note: This game can only be played on a PS3 system.

No, if Forbidden Siren (PS4) and Siren: Blood Curse (PS3) were Crossbuy, there would be a "purchased" / "erworben" tag on it in the PS Store listing and no option to buy the PS4 version:

And a PS4 version of "Siren: Blood Curse" ain't to find in the German PS Store, neither via PS4 nor via web browser. What you were rating was the old PS3 version, not a PS4 version.

Unfortunately Sony has phased down its Crossbuy strategy a lot since their competition got weaker / less threatening. :(

Let's hope that Microsoft's new crossbuy-strategy (Xbox/Win10) and Nintendo's (expected) unified library NX turns this trend around.

Do you have a screenshot where it shows PS4?

It doesnt say anything ps3 for me in the description, still it shouldnt be in the ps4 store if its just a ps3 game. Thats usually how it doesnt work

why should a ps3 page exist in the store for ps4? 

It says ps3 now

you can see it if you go to ps4 store > spiele > genres > andere > horror

but i could swear i saw something ps4 before like in the description. It still shouldnt be in the ps4 store if its a ps3 game, and we know that siren bloodcurse was rated in korea before

probably it is shown in the PS4 store due to a wrong tag in the shop-database. It still says 720p

Last Blade 2 was crossbuy this month