Correct way to set ByteOrderedPartitioner initial tokens

[Mateus Ferreira e Freitas]

I have 4 Nodes, and I'd like to store all keys starting with 'a' on node 1, 'b' 
on 2, and so on.My keys just start with a letter and numbers follow, like 
'a150', 'b1','c32000'.I've set the initial tokens to 61ff, 62ff ,63ff, 64ff 
.This does not seem to be the correct way.Thanks.