PoD is the fastest way to an ep 4 boss by far (Shambertin or rare spawn Kondrieu), a fast run can be as short as 10-15 minutes.
Government 9-8 is the fastest way to get to Saint Milion (still has chance of Kondrieu), the regular ep 4 boss, it's a 30-40+ minute quest.
Regular sub desert is highly unadvised, it can take you more than 1hr+ to reach the boss (Saint Milion or Kondrieu).
Currently Saint Milion drops the Blood Sword for summer event, a powerful spirit saber for forces. Pinkal id is the only confirmed id so far. He may drop the Soul Booster to make 100 souls on some other id, nothing confirmed yet. If you wish to get one of these, I recommend you complete the government quests up to 9-8 on normal to unlock for all difficulties, which should only take about 2 or so hours, regular sub-desert is not worth running. If you just want Kondrieu and don't care about the blood sword / soul booster do PoD.