You get the post time favorite. There are no refunds in the pick 4, even if the bet is made before the sequence starts.
http://www.nyra.com/belmont/handicapping/wagering-faq/
Pick Four, Pick Six, and Grand Slam scratches
For Pick Four, Pick Six and Grand Slam wagers, in the event of a late scratch, the bettor receives the post time favorite. There are no refunds or alternate selections. In the Pick Four, there is a mandatory daily payout of the entire pool. In the Pick Six, in the event that there are no tickets with six winners, a consolation payoff is paid to bettors with the most winners. There is a carryover of 75% of the net pool into the next day of the meet.
Pick Four, Pick Six, and Grand Slam-races off turf
When the condition of the turf course(s) warrants a change of racing surface in any legs of the Pick Four, Pick Six and/or Grand Slam races, and such change has not been made known to the public prior to the close of wagering for the Pick Four, Pick Six or Grand Slam, the stewards shall declare the changed leg(s) an "all win" race for Pick Four, Pick Six and Grand Slam wagering purposes only.