Explanation:

if 100 cats kills 100 mice in= 100 days;
Then , 100 cats will kill 1 mouse in = 1 days; => 1 cat will kill 1 mouse in = 100 days;
And, 4 cats will kill 1 mouse in = 25 days;
Hence, 4 cats will kill 4 mouse in = 100 days;