1. M. D. Hatch and C. R. Slack while working on sugarcane found four-carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis. 

2. It occurs in tropical and sub-tropical grasses and some dicotyledons.

3. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C₄ pathway and plants are called C₄ plants.

 Mechanism: 

1. CO₂ taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase. 

2. It is converted to another 4-C compound, malic acid. It is transported to the chloroplasts of bundle sheath cells. 

3. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO₂ in the cytoplasm. Thus, concentration of CO₂ increases in the bundle sheath cells. 

4. Chloroplasts of these cells contain enzymes of Calvin cycle. 

5. Because of high concentration of CO₂ , RuBP carboxylase participates in Calvin cycle and not photorespiration. 

6. Sugar formed in Calvin cycle is transported into the phloem. 

7. Pyruvic acid generated in the bundle sheath cells reenter mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP. 

8. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. Thus, C₄ pathway needs 12 additional ATP. 

9. The C₃ pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C₄ pathway requires 30 ATP. 

10. Thus, C₄ plants are better photosynthesizers as compared to C₃ plants as there is no photorespiration in these plants.