Suppose there's an official repo maintained called O with branches B1, B2 & B3.
One user, who has forked it onto his Github account, made another branch for himself called B4 and is publicly available.
I've also forked the same official repo but I want to fork that user's B4 branch also without affecting my original copy. 
I cannot fork the whole official repo again as I've made several custom branches for myself.
So, how can I fork a particular branch onto my Github repo?
 
     
     
    