Your open position is marked based on the current market price.
If you own 4,000 “Yes” shares and the “Yes” price moves from 25 to 40, the position value increases from approximately $1,000 to approximately $1,600 before fees, spreads, and any execution differences.
If the “Yes” price falls from 25 to 10, the same position value falls from approximately $1,000 to approximately $400.