In order to answer this question, a basic of scientific notation must first be achieved. Scientific notation is, quite simply put, a way of writing really large numbers in a smaller amount of space. It is a notation born out of laziness, if you will.
The idea is that instead of writing the whole number out, one will simply write a multiplication problem, and have the core number (that is, the number appearing first) in the form of a single digit.
As such, the number 4,000,000 would be 4 multiplied by one million, and one million can be written as 10 x 10 x 10 x 10 x 10 x 10, or 10^6. So, 4 million would be 4 x 10^6, making the power of 10 positive.
This makes sense, because if the power of 10 was negative, then the number would be instead 10/10/10/10/10/10, which would be much smaller. Since 4 million is a large number, we can know that the power of 10 will be positive.
Hope this helps :)
