Four inches is the common figure for a garage or shed slab, and a house slab on the clay in this corridor is an engineered element rather than a thickness. Around here the reinforcement and the soil preparation matter far more than the number of inches.
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The four-inch answer that dominates search results is right for the wrong slab. It applies to a detached garage, a shed, or a patio on stable ground. A house foundation on the soil in this market is a different problem. This corridor sits on Blackland Prairie clay, which is highly expansive: it swells as it takes on water and shrinks as it dries, moving whatever is built on it. A slab poured on that ground is not resting on something stable, it is spanning across something that moves. That is why residential foundations here are engineered, typically as post-tension slabs with tensioned cables running through them or as conventionally reinforced slabs with a beam grid, and why the design comes from an engineer working from a soil report rather than from a rule of thumb. What actually determines performance on this ground, roughly in order. Soil preparation. Moisture conditioning of the subgrade before the pour, so the clay goes in at a controlled moisture content rather than saturated or bone dry. This is the step most often shortchanged and the one with the largest effect. Reinforcement design. Post-tension cable layout, or rebar size and spacing with the beam grid, sized to the soil and the structure. Never cut or core a post-tension slab without locating the cables first. Drainage and moisture management after the pour. Downspouts discharging well away, grading falling away from the slab, and consistent perimeter moisture. Foundation watering during extended drought is ordinary practice here rather than folklore. Curing. Concrete gains strength through hydration, not drying, and a slab that loses moisture too fast in this heat cures weaker and cracks more. Keeping it damp during the first days is doing something real. For the simpler slabs where the four-inch answer does apply: four inches over a compacted base is standard for a garage or shed, five to six for a shop slab that will see vehicle or equipment loads, with reinforcement sized to the use. Permit requirements are set by the city the property sits in, or by the county for unincorporated areas, so confirm with that office before design.