First, you must solve for (4 x 10^3) and (2 x 10^8)
Think of the ^3 and ^8 as decimal places. Ten to the positive three means to move the decimal behind the 4, backwards the increase the number. So (4 x 10^3) would be equal to 4000. 3 decimal places back from 4.
Do the same thing for (2 x 10^8) and you get 200000000.
TLDR: (2 x 10^8) > (4 x 10^3) because (2 x 10^8) = 200000000 and (4 x 10^3) equals 4000.