From the CD website.

13. In the event a Pick-4 ticket designates a selection in any one or more races comprising the Pick-4 and that selection is scratched, excused or determined by the Stewards to be a non-starter in a race, the actual favorite, as evidenced by the amounts wagered in the final win pool of the race, will be substituted for the non-starting selection for all purposes, including pool calculations and payoffs. In the event the win pool total for two or more favorites is identical, the substitute selection shall be the betting interest with the lowest program number.