Quark-lepton interchangeability at high energy: $\left[\mathrm{SU}\right(3)\mathrm{}{]}^4$ quartification

We propose that quarks and leptons are interchangeable entities in the high-energy limit. This naturally results in the extension of $\left[\mathrm{SU}\right(3)\mathrm{}{]}^3$ trinification to $\left[\mathrm{SU}\right(3)\mathrm{}{]}^4$ quartification. In addition to the unbroken color ${\mathrm{SU}\left(3\right)\mathrm{}}_q$ of quarks, there is now also a color ${\mathrm{SU}\left(3\right)\mathrm{}}_l$ of leptons which reduces to an unbroken ${\mathrm{SU}\left(2\right)\mathrm{}}_l.$ We discuss the natural occurrence of ${\mathrm{SU}\left(2\right)\mathrm{}}_l$ 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 ${10}^{34}–{10}^{36}$ yr.