Modular exponentiation is an important operation in public-key cryptography. The Common-Multiplicand-Multiplication (CMM) modular exponentiation is an efficient exponentiation algorithm. This paper presents a novel method for speeding up the CMM modular exponentiation algorithm based on a Modified Montgomery Modular Multiplication (M4) algorithm. The M4 algorithm uses a new multi bit scan-multi bit shift technique by employing a modified encoding algorithm. In the M4 algorithm, three operations (the zero chain multiplication, the required additions and the nonzero digit multiplication) are relaxed to a multi bit shift and one binary addition in only one clock cycle. Our computational complexity analysis shows that the average number of required multiplication steps (clock cycles) is considerably reduced in comparison with other CMM modular exponentiation algorithms.