If the ratio of white to blue is 4:3, then the total number of pens must be a multiple of (4+3) = 7. The smallest number that is both greater than 40 and is divisible by 7 is 7*6 = 42.
Therefore, the minimum number of pens in Jake's drawer is 42. (In that case, there would be 24 white pens and 18 blue pens.)