Empirical studies have shown that plant photosynthetic responses to environmental change can vary over time due to acclimation, but acclimation responses are often not included in Earth System Models (ESMs). Photosynthetic least cost theory can be used to develop models of photosynthetic acclimation that are simple and testable. The theory is based on the idea that, optimally, plants will acclimate to maintain the fastest rate of photosynthesis at the lowest water and nutrient use. Formulations of this theory have been developed for C₃ plants, but not C₄ plants, which account for over 20% global photosynthesis and are over-represented among widely grown crops. Here, we use photosynthetic least cost theory to derive a model for C₄ photosynthetic acclimation to above-ground abiotic conditions. We then compare our model’s responses to a similar model of C₃ photosynthetic acclimation and find that C₄ photosynthesis has the highest simulated advantage over C₃ photosynthesis in hot and low CO₂ environments. We find that this advantage predicts C₄ abundance globally, but that the shallower CO2 response of C₄ as compared to C₃ photosynthesis will reduce C₄ plant competitiveness under future conditions, despite higher temperatures. We also show that an acclimated model predicts similar or faster rates of C₄ under all conditions than a model that does not consider acclimation, suggesting that ESMs are underestimating future C₄ carbon uptake by not including acclimation. Our model is designed for easy incorporation into such ESMs.