If you look at the RMD hash (739437bb3dd6d1 9 83e66629c5f08c70e52769371), you will see that for your combination 1BY8GQbnueY 14 hex numbers are used, plus one more digit from 4 to a. In total, this gives 2^56 bits.
And how did you come to the conclusion that in the 66-bit range there are only 4 combinations?