C3 is Calvin cycle which involves only formation of C3 acid i.e. PGA and primary CO2 acceptor is RuBP.
Whereas C4 is hatch and slack pathway which involve formation of OAA and primary CO2 acceptor is PEP.
C4 pathway also follow C3 cycle but in C4 pathway C3 acid is formed in bundle sheath where as in C3 pathway C3 acid is formed in mesophyll.
