In my experience, there are plenty of times when more than 4 digits are handy, especially for creation of sub-accounts.  For example, let's say for a nonprofit organization you have donor restricted cash in Bank A.  If the account number for Bank A is 1010, the savings account at that bank is numbered 20, the parent account for donor restricted cash is 11, and the donor's account is 1234,  then the account number would be 101020111234.  Ideally for clarity, if QBO allows it, breaking up the account number with dashes (-) or periods (.) would be helpful—1010.20.11.1234 or 1010-20-11-1234.

     There might be a simpler way to do this by relative linking, but QBO probably lacks such flexibility.