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The Rayleigh criterion gives resolution as k1 x lambda / NA. With 193 nm immersion at NA 1.35 the single-exposure limit is around 38-44 nm half-pitch, which is why sub-20 nm nodes needed double and quadruple patterning until EUV at 13.5 nm arrived.
Rayleigh resolution
R = k1 x lambda / NA; multiple patterning divides the achievable pitch
Water between lens and wafer raises the refractive index, allowing NA above 1 — impossible in air — which directly improves resolution.
Each extra exposure adds mask cost, cycle time and overlay error. Quadruple patterning can make a single layer cost four times as much.