Yes, forfeited salary is credited to the team's salary cap for the next league year. See CBA, p. 96, section 6 (iv). But it is within the discretion of the team to decide whether the player must forfeit the salary or previously paid portion of the salary cap.
According to the article that John Clayton wrote, if Marshawn continues his hold-out through all 4 preseason games, the fines will total $2.9M... fwiw.
And after that, each regular season game check is for $312,500... so to get the total to the $4.5M you posted would also entail him holding out through the first 5 regular season games.