In $\mathrm{C}_4$ plants with Kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. The vascular bundles are surrounded by several layers of specialized large sized cells called bundle sheath cells. The $\mathrm{C}_4$ cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells.
In $\mathrm{C}_4$ plants, the first step of carbon dioxide fixation takes place in the chloroplasts of mesophyll cells containing PEP, while bundle sheath cells contain RuBisCO.
Related Theory
The $C_4$ plants consume more energy (i.e., two additional molecules of ATP) during the fixation of carbon dioxide. The mesophyll cells of the $C_4$ plants are closely packed which provide smaller area for better utilization of the available water. This also reduces the intensity of solar radiations. This makes the $C_4$ plants better adapted to the conditions of tropical climate. They do not show photorespiration. Thus, their photosynthetic rate is quite high.
Caution
Bundle sheath cells are absent in $\mathrm{C}_3$ plants.