Think of the common multiple of 4 and 5
Multiple of four is 4, 8, 12, 16, 20, 24, 28, 32, ...
Multiple of five is 5, 10, 15, 20, 25, 30, ...
The first common multiple is 20 so we can aim for the Fire Ant and Crab Spider to be magnified to 20 millimeters
Fire Ant is to be magnified 20÷4 = 5 times
Crab Spider is to be magnified 20÷5 = 4 times