You are touching two slightly different ideas here, which are getting mixed together:

1. “Shashwat Tirth” (eternal tirths / places)
2. The 24 Tirthankars on Ashtapad and the 4/8/10/2 arrangement

Let’s separate them clearly.

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1. “Shashwat” – what is really shashwat in this context?

In Jain tradition, “Shashwat Tirth” normally means some pilgrimage places are eternal, not that some Tirthankars are “shashwat” and others are not.

The commonly accepted Shashwat Tirths are:

1. Shatrunjay (Palitana)
2. Sammed Shikharji
3. Girnar
4. Mount Abu
5. Ashtapad (where Bhagwan Rishabhdev attained moksha)

You can read more about Shashwat Tirths here:

So:

• Ashtapad is a Shashwat Tirth (place).
• The 24 Tirthankars themselves are not divided into “4 shashwat Tirthankars + 20 others” in the scriptures.
• All 24 of our present chovisi are Tirthankars of this avasarpini; none of them are called “shashwat Tirthankars” in that technical sense.

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2. Ashtapad and the 24 Tirthankars

According to Jain tradition:

• Bhagwan Adinath (Rishabhdev) attained moksha on Ashtapad.
• His son Bharat Chakravarti later built a very grand temple / structure on Ashtapad with idols of all 24 Tirthankars.

About the arrangement of these 24:

a) Scriptural / traditional point

Scriptures and older traditions mainly say:

• There is a chovisi (24 Tirthankars) installed on Ashtapad.
• They are arranged according to directions (east, south, west, north) and places on the mountain, in various pujas and stavans.

But:

• They do not clearly state a doctrinal division like “4 shashwat + 20 normal” Tirthankars.
• The “shashwat” idea is properly attached to tirths (places), not to a subset of Tirthankars.

b) Where does 4 / 8 / 10 / 2 come from?

Modern descriptions of Ashtapad models / replicas often mention that the 24 idols are in different size groups, for example:

• 2 idols – extra large
• 4 idols – large
• 8 idols – medium
• 10 idols – small

This kind of 2 / 4 / 8 / 10 (or your 4 / 8 / 10 / 2) partition is:

• About idol sizes and artistic design in particular temples or models.
• It is not a theological rule from Agams like “these 4 are shashwat Tirthankars and the rest 20 are different.”

Different temples, replica-mountains, or puja-books may choose their own way of:

• deciding which Tirthankar is big/small, or
• which direction they face, or
• how they are grouped for puja.

These are vyavahaar / practical-arrangement traditions, not deep tattva-based categories of Tirthankars.

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3. So what is the “basis” of 4/8/10/2?

In short:

• From the angle of Jain doctrine (tattva and Agam):

There is no core philosophical division of the 24 Tirthankars of Ashtapad as “4 shashwat + 20 others” with a 4/8/10/2 rule.

• From the angle of temple / model design:

The numbers like 2–4–8–10 etc. come from: - How many idols of which size an artist or acharya decided to install, or - How a particular Ashtapad replica was planned.

So the basis is architectural / artistic and ritual, not a “cosmic law” about the Tirthankars themselves.

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4. Key takeaway in very simple words

• Ashtapad is a Shashwat Tirth (eternal holy place).
• All 24 Tirthankars are equally siddha-worthy and perfect; Jain shastra does not say only 4 of them are “shashwat Tirthankars.”
• The 4/8/10/2 type arrangements you see are human-made patterns (sizes, placement, directions) used in certain Ashtapad temples or models, not a basic rule of Jain philosophy.

If you are looking at a particular temple or Ashtapad model, their own booklet or board will usually explain how they chose that specific grouping.