Click the link and it should take the to the PS Store home page. Type into the search bar "tiny brains" and 4 games should pop up. First add and confirm the PS3 demo (timed trial on search results page), then click on the PS4 game and also add that demo (click on the main game and under add to basket should be a download demo option), then go back to the search results page and you should see the PS3 full game upgrade drop to 0.00, go add it to your basket and confirm at checkout, you will also automatically recieve the PS4 version as part off cross buy.
Enjoy playing your free game
You can also click this direct search link but it could not work depending on your region, this glitch works on all regions as long as the game is in your region, the link below is for the UK region but with others just go the home page that i put in the deal link and search for the game.store.playstation.com/