$No. of superkeys = 2^{M - N}$
where, M = Total number of attributes, N= number of attributes in given candidate key
In this question, M = 4 , N = 1, Therefore, no. of superkeys = $2^{4-1} = 2^{3} = 8$
Note that if multiple candidate keys are given we need to consider set theory to find the superkeys possible.
For example, R(A1,A2,A3,......,An) and Candidate keys = {A1,A2}
then superkeys, $SKs = SK(A1) + SK(A2) - SK(A1A2)= 2^{n-1}+2^{n-1}-2^{n-2} $