  We consider four-component Dirac operators on domains in the plane. With
suitable boundary conditions, these operators describe graphene quantum dots.
The most general boundary conditions are defined by a matrix depending on four
real parameters. For operators with constant boundary parameters we show that
the Hamiltonian is unitary equivalent to two copies of the two-component
operator. This allows to extend the known results for this type of operators to
the four-component case. As an application, we identify the boundary conditions
from the tight-binding model for graphene that give rise to a block-diagonal
operator in the continuum limit.
