The process which makes major difference between C3 and C4 plants is
C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO2 concentration in the chloroplast drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.
The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO2 high and oxygen low in the chloroplast where the Calvin cycle operates. The class of plants called C3-C4 intermediates and the CAM plants also have better strategies than C3 plants for the avoidance of photorespiration.