Seeking the probability that the test A drug is more effective

In order to treat a certain disease, two new drugs, A and B, have been developed, hoping to know which new drug is more effective, and animal experiments are conducted for this. The test plan is as follows: Two white rats are selected for the comparative test of the efficacy in each round. For two white mice, one is randomly selected to be given drug A and the other is given drug B. After the results of one round of treatment are obtained, the next round of trials will be arranged. When one of the drugs cured 4 more white rats than the other, the experiment was stopped, and the more effective drugs were considered to be more effective. In order to facilitate the description of the problem, it is agreed that for each round of the experiment, if the rats given drug A are cured and the rats given drug B are not cured, then drug A will get 1 point, and drug B will get -1 point; if the mice given drug B are cured If a mouse that is cured and administered with drug A is not cured, drug B will get 1 point, and drug A will get -1 point; if both are cured or not cured, both drugs will get 0 points. The cure rates of A and B drugs are respectively. Marked as a and b.