Quark-lepton interchangeability at high energy: [SU(3)]⁴ quartification

We propose that quarks and leptons are interchangeable entities in the high-energy limit. This naturally results in the extension of [SU(3)] ³ trinification to [SU(3)]⁴ quartification. In addition to the unbroken color SU(3)_q of quarks, there is now also a color SU(3); of leptons which reduces to an unbroken SU(2)_l. We discuss the natural occurrence of SU(2)_l doublets at the TeV energy scale, which leads remarkably to the unification of all gauge couplings without supersymmetry. Proton decay occurs through the exchange of scalar bosons, with a lifetime in the range 10³⁴-10³⁶ yr.