Answer

In Jain tradition, 4 upvas is generally called “Chau-vihar Upvas” or “4 Upvas”, not Atham.

Atham usually refers to a tapasya of 3 consecutive upvas.

Explanation

The word Atham is traditionally connected with the 8th tithi or the practice associated with three-day fasting observances in Jain tradition. In common Jain usage:

• 1 upvas = one day fast
• 2 upvas = Chhath
• 3 upvas = Atham
• 4 upvas = four upvas / sometimes called Chauvihar tapas depending on local usage

So if someone is calling 4 upvas “Atham,” it may be due to local or family usage, but traditionally Atham means 3 upvas, not 4.

Takeaway

Atham = 3 upvas in standard Jain usage. 4 upvas should simply be understood as four consecutive upvas, unless a specific local tradition uses a different name.