Superkeys of this relation

normalizationprimary-key

I am trying to find the superkeys of this relation, but I am having troubles finding out how many superkeys there are and exactly what they are. I figured out that the candidate keys were {A},{B},{C},{D}.

Here is the relation:

R(A,B,C,D)

Functional Dependencies: 

A->B
B->C
C->D
D->A

Candidate keys: {A},{B},{C},{D} (from what I figured out)

Can someone please help me find the superkeys, and how exactly to find them?