Calculating probablities

[Felex]

I need some help figuring my odds in a game we created in my regular Thursday Night game. Players are dealt 4 down cards and seven cards are spread on a board. The cards are flipped up in 4 rounds, 2-2-2-1. You have to throw any card that comes up on the board (any board card that we have already seen gets replaced with another card from the deck). So at the end of the game 6 of the 13 possible cards will have thrown out. How do I calculate my odds toward the end to determine whether to call a bet?

For example, say I have two cards just before we the last card is about to be turned. Say I have A J (11 or 21). I am facing one player with one card, one with three and another with four. The betting is usually 20 limit betting. What are my odd's of a Jack coming in on the last card giving me the lock-low with the Ace? This a high-low point game (blackjack points). Is it 7:1?

This game is a lot of fun but I can't get a handle on when to stay in and when to muck it. Any insight is appreciated.