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The optimization and application of new functional materials depends critically on our ability to manipulate the charge carrier density. Despite predictions of good n-type thermoelectric performance in the quaternary telluride diamond-like semiconductors ( e.g. Cu 2 HgGeTe 4 ), our prior experimental survey indicates that the materials exhibit degenerate p-type carrier densities (>10 20 h + cm −3 ) and resist extrinsic n-type doping. In this work, we apply the technique of phase boundary mapping to the Cu 2 HgGeTe 4 system. We begin by creating the quaternary phase diagram through a mixture of literature meta-analysis and experimental synthesis, discovering a new material (Hg 2 GeTe 4 ) in the process. We subsequently find that Hg 2 GeTe 4 and Cu 2 HgGeTe 4 share a full solid solution. An unusual affinity for Cu Hg and Hg Cu formation within Cu 2 HgGeTe 4 leads to a relatively complex phase diagram, rich with off-stoichiometry. Through subsequent probing of the fourteen pertinent composition-invariant points formed by the single-phase region, we achieve carrier density control ranging from degenerate (>10 21 h + cm −3 ) to non-degenerate (<10 17 h + cm −3 ) via manipulation of native defect formation. Furthermore, this work extends the concept of phase boundary mapping into the realm of solid solutions and clearly demonstrates the efficacy of the technique as a powerful experimental tool within complex systems.