The seeding of the cards on the street make it possible that there are only few PhD cards available (some are still on the draw deck)?
So it's possible that there is only 1 PhD card on the street and with 4 players only 1 player has the chance to grab it with the buy action?
As all PhD cards are on the bottom of the draw deck, it is possible, that first only one shows up.
As is true for all other cards, too, only one player can buy each single card. So the first player in turn order may buy the PhD and takes it from the street.