C₄ carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when supplied with ¹⁴CO₂, incorporate the ¹⁴C label into four-carbon molecules first.

C₄ fixation is an addition to the ancestral and more common C₃ carbon fixation. The main carboxylating enzyme in C₃ photosynthesis is called RuBisCO, which catalyses two distinct reactions using either CO₂ (carboxylation) or oxygen (oxygenation) as a substrate. The latter process, oxygenation, gives rise to the wasteful process of photorespiration. C₄ photosynthesis reduces photorespiration by concentrating CO₂ around RuBisCO. To ensure that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen, C₄ leaves generally differentiate two partially isolated compartments called mesophyll cells and bundle-sheath cells. CO₂ is initially fixed in the mesophyll cells by the enzyme PEP carboxylase which reacts the three carbon phosphoenolpyruvate (PEP) with CO₂ to form the four carbon oxaloacetic acid (OAA). OAA can be chemically reduced to malate or transaminated to aspartate. These intermediates diffuse to the bundle sheath cells, where they are decarboxylated, creating a CO₂-rich environment around RuBisCO and thereby suppressing photorespiration. The resulting pyruvate (PYR), together with about half of the phosphoglycerate (PGA) produced by RuBisCO, diffuses back to the mesophyll. PGA is then chemically reduced and diffuses back to the bundle sheath to complete the reductive pentose phosphate cycle (RPP). This exchange of metabolites is essential for C₄ photosynthesis to work.

On one hand, these additional steps require more energy in the form of ATP to regenerate PEP. On the other hand, concentrating CO₂ allows high rates of photosynthesis at higher temperatures. Higher concentration overcomes the reduction of gas solubility with temperature (Henry's law). The CO₂ concentrating mechanism also maintains high gradients of CO₂ concentration across the stomatal pores. This means that C₄ plants have generally lower stomatal conductance, reduced water losses and have generally higher water-use efficiency. C₄ plants are also more efficient in using nitrogen, since PEP carboxylase is much cheaper to make than RuBisCO. However, since the C₃ pathway does not require extra energy for the regeneration of PEP, it is more efficient in conditions where photorespiration is limited, typically at low temperatures and in the shade.