Symmetric Localization of ν_{tot}=4/3 Fractional Topological Insulator Edges

Motivated by the recent twisted MoTe_{2} experiment [Wang et al., Magnetic Signatures of a Putative Fractional Topological insulator in twisted MoTe_{2}, arXiv:2601.18508], we develop a disordered interacting edge theory of a fractional topological insulator at ν_{tot}=4/3, consisting of two time-reversal-conjugated ν=2/3 fractional quantum Hall states. For an S_{z}-conserving edge, we uncover three distinct phases with two possible conductance values per edge in the long-edge limit: 2/3(e^{2}/h) and 4/3(e^{2}/h). In the presence of S_{z}-changing perturbations (e.g., Rashba spin-orbit coupling), an interaction-induced insulating edge state can emerge without breaking time-reversal or charge-conservation symmetry, corresponding to the absence of a topologically protected edge state. We show an exact mapping (with a special choice of parameters) to a noninteracting fermionic theory exhibiting Anderson localization, and the weak-coupling phase diagrams are also constructed, showing that symmetric localization can emerge regardless of other S_{z}-conserving perturbations. Our results showcase an explicit, experimentally relevant example that the edge-state two-terminal transport can yield false-negative results in identifying the ν_{tot}=4/3 fractional topological insulators.