4n * n^3
n is the same as writing n^1. therefore, using some maths laws which i cant remember the general topic name for, n^a * n^b = n^a+b
so n^1 * n^3 = n^4.
the 4 infront of the first n is just multiplying it by four, which is carried over to the end result of 4n^4.
(apologies for the terribly described answer, i was tired when writing it)