To evaluate the problem with the 4m TRH, I swapped it with 5.5m between ~0915-0925.  It appears that S/N 10 (the one formerly at 4m, now at 5.5m, that we swapped in as our "spare" a few days ago) has a bias.  We'll leave it in this configuration, since 5.5m RH is not a critical measurement.