I don't trust any of those guys, so a 4th loss is possible. I'd say the odds are at least 10%
Thats the way either you or I would figure the bet, but a sports book is done differently.
A proposition bet is forwarded by the book ie Cards over the Sox in 4 and a line of probablility is assigned to it which, in the mind of the odds setter, will attract bettors to each side in quantities to balance the payouts. After it is on the board the odds start to vary as people place money on it. The books don't look to make money on the bet (if they do the odds are really a failture), just the vig, or cost of the bet. In a perfect world for a book, a proposition bet would have enough bettors on each side of the line to balance out and the books collects a fraction of each bet.