[dragonwriter]

> Why isn't there a better source for this than "four people"?

The same reason that (except for actual indictments and subsequent court filings, and the extraordinary fact of the appointment of a special prosecutor) we get most—often later confirmed as accurate—info on the Mueller probe through similar sources outside DoJ: the DoJ doesn't leak much but a criminal investigation naturally touches people outside the DoJ that are less prone to avoiding leaking. Those can be people in the agency giving a criminal referral, which this reporting suggests would likely have come from the CFTC, or they can be people connected with witnesses (including, but not limited to, those who have been given notice indicating that they are subjects or targets of the investigation.)