The question cannot be answered without further information. If I understand the question correctly, the range of the sequence is either $\{0,2,4,6,8\}$ or $\{2,4,6,8\}$, depending on whether your definition of natural number includes $0$; mine does, but yours may not. However, there is no way to tell what the domain is unless your textbook or instructor has established some convention. If the sequence is one-to-one, listing each number in the range only once, the two most likely conventions would make the domain either $\{0,1,2,3,4\}$ or $\{1,2,3,4,5\}$ if the range is $\{0,2,4,6,8\}$, and $\{0,1,2,3\}$ or $\{1,2,3,4\}$ if the range is $\{2,4,6,8\}$.