One of the key problems in bringing nanotechnology to commercial reality is in finding design methodologies that permit such devices to be combined into computationally useful circuits with favorable density gains over ACMOS, but with minimum assembly complexity. This paper develops one such structure for one of the more rapidly maturing nanotechnologies termed QCA. The "4-Diamond" Structure is shown to have interconnect and logical completeness properties similar to a classical FPGA cell, but which if implemented with molecular QCAs on a tiled DNA substrate, has the potential for self-assembly into large structures. When applied to a sample design problem, it has density at least equal to what might be expected from end of the roadmap silicon, but with the potential for significant further improvements.