I believe the Liberator is the one you're referencing, and you're right it was available through the 4th of July, however, to my knowledge hasn't been available in the PS4 or X1 version unless doing a race or something. (not to purchase...yet...maybe 7/4/15?)
The one that is available to purchase is the Marshall, for returning players. However isn't available if you hadn't played the x360 or PS3 version. Before I borrowed or rented the last gen version to see if it triggers the ability to purchase it in the PS4 version, I was hoping someone else here has already tried and can confirm if it works or not. The same goes for the other unlocks in Single player and Online.
I think Brad being a reviewer, will do it. Now that he's officially hooked on the series and knows quite a bit about the lore and character builds, I think he'll be in the best position to do it justice. I'd say Rorie then Vinny for a close 2nd and 3rd though.
I would love to see another Vinny run through with maybe Rorie and Brad chiming in from time to time, next year, would be great.