Sulfur is important to planetary habitability, but the early sulfur cycle is poorly understood. In particular, S[IV] species (HSO₃^-, SO₃^2-), derived from volcanogenic SO₂, are critically invoked in recent proposals for origins-of-life chemistry and also influence atmospheric sulfur haze formation, but their abundance in early natural waters is unclear. Here, we combine new laboratory constraints on the kinetics of S[IV] disproportionation with a novel aqueous photochemistry model to estimate the concentrations of S[IV] in natural waters on prebiotic Earth. We show that S[IV] disproportionation is slow in pH≥7 waters, with timescale T≥1 year at room temperature, meaning that S[IV] was present in prebiotic natural waters. However, we also show that photolysis of S[IV] limits [S[IV]]< 100 μM in global-mean steady state. Marine S[IV] was sub-saturation with respect to atmospheric SO₂, meaning that climate-altering, UV-attenuating sulfur hazes did not persist on prebiotic Earth. [S[IV]] was much lower in natural waters compared to the concentrations generally invoked in laboratory simulations of origins-of-life chemistry (≥10 mM), meaning further work is needed to confirm whether S[IV]-dependent prebiotic chemistries discovered in the lab could have realistically functioned in nature. [S[IV]]≥1 μM in terrestrial waters for: (1) SO₂ outgassing ≥20× modern, (2) pond depths <10 cm, or (3) UV-attenuating agents present in early waters or the prebiotic atmosphere. Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry experiments in understanding the emergence and maintenance of life on early Earth.