Prepare: Line a 13-by-18-inch rimmed baking sheet with parchment rated for 450°F. Have a deep oven-safe metal roasting pan that covers all four loaves without touching them; check its fit first. Reserve 12 g bread flour for shaping.

Mix: Stir the ripe sourdough starter into the water. Add 400 g bread flour and the salt. Squeeze and stir until no dry flour remains. The dough should be sticky and cohesive, holding a rough mound. Cover.

Bulk ferment: Allow 6 hours at 75–78°F, checking from 4 hours. At 30, 60, and 90 minutes, use a scraper to fold the dough inward from four sides; cover between sessions. These rests belong to bulk fermentation. Continue until 60–75% larger, domed, bubbly at the edges, and lighter. Cooler dough may take longer.

Shape: Allow 20 minutes for the bench rest. Dust the counter with half the reserved flour. Divide into four approximately 195 g pieces before dusting, form loose logs, cover, and rest. Distribute the remaining reserved flour over sticky surfaces. Gently flatten each log, fold the upper third inward, then fold lengthwise and pinch closed. Roll to 9 inches. Arrange seam down, parallel to the sheet’s short sides, with 2 inches between loaves.

Final proof: Allow 90 minutes at 75–78°F, checking from 60 minutes. Cover loosely until puffy; a fingertip dent should return slowly and remain faintly visible. Meanwhile, preheat the oven to 450°F with a middle rack. Uncover and score each loaf with three overlapping diagonal cuts about 1/4 inch deep.

Bake: Bake for 25 minutes at 450°F: cover with the inverted roasting pan for the first 15 minutes. Wearing oven mitts, lift the cover away from your face to release steam. Bake uncovered for the remaining 10 minutes, until deep golden and approximately 207°F inside. The cover traps dough moisture; add no water.

Cool: Transfer immediately to a wire rack and cool for 40 minutes before slicing so the crumb can set.