In ignorance of cricket, I will assume that a century means exactly $100$ runs.
Let's rephrase the question in terms of money. In how many ways can we have $100$ dollars in $4$ dollar bills and/or $6$ dollar bills? (It looks as if I don't know much about money either.)
The argument will be easier to grasp if we solve the equivalent problem of producing $50$ dollars in $2$ dollar and/or $3$ dollar bills. It is clear that we must use an even number of $3$ dollar bills, $0$ to $16$, and then we can make up the rest of the $50$ dollars with $2$ dollar bills. There are $9$ (not $8$) even numbers between $0$ and $16$ inclusive.
Note that if the order in which the types of scores were made matters, then the answer is hugely larger than $9$. Would you view $4$ then $6$ then $4$ as different from $4$ then $4$ then $6$?
Added: Derek Holt remarks that if one gets to $98$ with $4$'s and/or $6$'s, and then gets a $4$ or a $6$, one is still deemed to have scored a century with $4$'s and $6$'s. The same method as the one used above shows that there are $9$ ways to reach $98$. That interpretation gives an additional $18$ possibilities, for a total of $27$.