  We prepare a gate-defined quadruple quantum dot to study the gate-tunability
of single to quadruple quantum dots with finite inter-dot tunnel couplings. The
measured charging energies of various double dots suggest that the dot size is
governed by gate geometry. For the triple and quadruple dots we study
gate-tunable inter-dot tunnel couplings. Particularly for the triple dot we
find that the effective tunnel coupling between side dots significantly depends
on the alignment of the center dot potential. These results imply that the
present quadruple dot device has gate performance relevant for implementing
spin-based four-qubit systems with controllable exchange couplings.
