Rockstar have stated that transferring data from the PS3 version of the game over to the PS4 version will happen for the online multiplayer. There has not been any confirmation of a ‘cloud save’ feature being added to the game.
Singe multiplayer data is stored on the Rockstar servers. It should be a painless process to log in with the PS4 version of the game and continue playing on with the character you had on the ps3 version of the game. Along with the official announcement of GTA 5 for PS4, PC and Xbox One there were confirmations that multiplayer data will be able to transfer from last gen to current gen.
“Gamers who invested plenty of hours into the game on the previous generation consoles will be happy to know that they can transfer your existing progress and multiplayer characters.”
To make things even better it will also be possible to transfer data between different platforms.
“As with the PS4 version, players will have the option of carrying over their GTA Online save data from either PlayStation 3 or Xbox 360 to any of the newly supported platforms.”