How is key length of feistel cipher calculated? In Feitsel Algorithim

If our block size is n, then our key length would be n x 2^n .

But this does not make sense for me. 
For example: Let's assume that we have 4 bit block size. If our block size is 4, how can the key length be 64 bit?
You have some confusion here: The formula N * (2 ** N) for key size is for ideal block ciphers that select one of (2 ** N)! permutations. Shortly after that formula, your book starts to go into the Feistel construction (emphasis is mine):

The Feistel Cipher
Feistel proposed [FEIS73] that we can approximate the ideal block cipher by utilizing
the concept of a product cipher, which is the execution of two or more simple ciphers
in sequence in such a way that the final result or product is cryptographically stronger
than any of the component ciphers. The essence of the approach is to develop a block
cipher with a key length of k bits and a block length of n bits, allowing a total of 2 ^ K possible transformations, rather than the 2 ^ N! transformations available with the ideal
block cipher

So the idea is to compromise on the huge key length and sacrifice the ability of picking one of all possible permutations, to utilize a smaller key that can select one of enough possible permutations. A cipher will typically use a key length that is equal to it's target block size, to simplify the key addition layer.