Although we don't know everything about how PS Plus will function on the PlayStation 4, we do know that it is definitely coming. So far, it will even be a part of the same subscription cost alongside Vita and PS3. Sony previously revealed that launch title DriveClub would be the first free game for PS4 PS Plus users.
But is this version as complete as the other existing Instant Game Collection titles? Unfortunately, no. An interview was conducted by PlayStationer with Evolution Studios which helped explain what is missing:
"The PS Plus version will be the exact same as the full version, except that it may be missing a few assets such as cars or tracks, that will be found in the full version. However the online multiplayer, the asynchronous challenges and the single player campaign will all be present in the PS Plus version. The PS Plus version gives us to the biggest open community, day one."
This means that you've got the full multiplayer experience but certain items are going to be kept from PS Plus users. They may choose to upgrade the free copy to the "full" digital version by, well, purchasing it. Apparently there is no way to simply buy the few items that are missing as DLC on their own, only the entire game. Perhaps we will see this parceling out of the game into free and paid versions more with PS4 PS Plus.