1) Do you have to use at least one pill per card, or can you use 8 with one card and 0 with the other?
2) Do you want to know the probability if both sides play randomly?
3) Or are you looking for a strategy that yields the highest probability for winning with your level 4 card, and you're wondering what probability is associated with that strategy?
4) Or are you looking for a strategy that yields the highest score (in terms of wins with the remaining cards), and given that strategy, you want to know the probability of the level 4 card winning?
5) Or are you looking for something else?
Assuming 3 or 4, are you assuming that the opponent optimizes his strategy? If so, what is the opponent's goal? Maximize wins with the remaining cards?
You need to more specific with your statement of the problem.