rqt I believe 4 days is much too short - particularly for problems that may easily take more than 4 days to sort out
persco It is enough days for the to and fros between helper and helpee
harrrrrry BA setters should initially look at threads that are 4 days old, do not yet have a BA and have had no input from the OP for at least 2 days
Just want to clarify here that the current process is that BA's will not be set (except by thread authors) until there has been at least 4 days since the last reply in the thread. It is absolutely not done based on when the thread was created, nor is it based on when the OP last replied (unless the OP was the last reply).
Back when 4 days was originally chosen, it was based on a chunk of research into a range of factors that showed that once a thread was 4+ days without a reply, the BA was either already present (but not set) or the thread was in need of resurrecting (which can and does happen from time to time).