How much dry cake mix, water, oil and eggs to use for two 4in round cakes?
I'll assume you want your 4" round cakes to be the same height as the two 8" rounds the box would make.

The area of two 8" rounds is 2*pi*4^2 = 32pi. The area of two 4" rounds is 8pi, or 1/4 as much area. So you only need 1/4 of the box, and 1/4 of the ingredients you'd add to it.

So, if you have a scale, you could measure 3.8oz of the cake mix, and otherwise you'll need to measure the whole thing by volume and take a quarter of it. Then you'll need 1/4 cup of water, 1/8 cup of oil, and... 3/4 of an egg, which you can probably round up to an egg. If you really want to be careful, you can also just beat an egg in a small bowl or measuring cup, and then spoon out 1/4 of it to leave 3/4 to use.