• A->>B is an MVD.

• A relation R is in 4NF. If whenever a nontrivial MVD is introduced its LHS should be a superkey

• An MVD is trivial if A⊇B or A∪B=R.

• Since A∪B=R it is a trivial MVD thus there is no nontrivial MVD thus this relation is in 4NF

So A is not actually a superkey. It is just that A->>B is trivial.