Update: Several users are reporting that although the released app (which is a simple hardcoded proxy) lets you download and install the Uncharted 4 Beta, The Beta itself will not accept to run unless you have a savegame of the Uncharted Nathan Drake collection on your PS4 system. This makes sense that Naughty Dog would implement that kind of check, and means the app that was released is pretty much useless, unless you want to trick your friends into thinking you got into the beta.
Th error message when you try to play the game. TY @GaetanFcp
The initial article:
Xmax katsu just released the Uncharted 4 Beta unlock application, a tool that lets you download and play Uncharted 4 now even if you’re not part of the Beta.
Xmax katsu recommends creating and using a new user on the PSN for the sake of trying this. It is possible Sony could take some preventive measures against accounts using this trick, and as such, using your main account is probably not a wise idea.
Of course, nobody in their right mind should use this tool ;). Do it at your own risk.