Dear student,
C4 pathway guarantees only carboxylase activity of Rubisco, to avoid losses due to photorespiration ( oxygenase activity of Rubisco). C4 pathway contains additional steps so, have extra energy requirement also. C4 pathway requires 30 molecules of ATP( 12 additional ATPs), whereas C3 pathway requires only 18 of ATP for the synthesis of one molecule of glucose. Sucrose contains one molecule of glucose and one molecule of fructose. Therefore for one molecule of sucrose 60 ATPs are required and for two molecule of sucrose 120 ATPs are required.
Regards