To compare two quantities, you should use a ratio (divide both numbers by the same number) to be able to compare more easily. To give the second quantity a value of one, you will divide that number by itself: 4/4 = 1. You will then also use this number (4) to divide the other number:
If you're comparing 2 to 4, divide both numbers by 4:
2/4 = 1/2
4/4 = 1
1 is clearly larger than 1/2, so 4 will be larger than 2.