On this page: http://www.ign.com/w...e?objectid=6862
a few vehicles are listed as included with the "Last Man Standing" update. According to the page: "As with previous updates, these Vehicles are also available in GTA 5 single-player mode for free via a player's Garage."
When I played the game, the garage had an option for accessing something like "Special Vehicles", but there was only a single car available (I don't remember the name).
The way that the site words it, it seems that there should be those 4 vehicles plus other vehicles from other updates available in the garage.