I think they meant its L3, in few places, maybe someone knowing a little bit less would have called L3 as L4.
L4 or TBT as per NSE domestic tariff is
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fat jayegi bhai
and thats way too much data anyway.
What you mentioned about prop trading, could be, but this data can only be consumed within co-lo.
co-lo costs are also very expensive.
Official blog calls it L3
https://zerodha.com/z-connect/featured/introducing-20-depth-or-level-3-data-beta-on-kite
L3 rates as per latest doc are: ( These seem to include tax )
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https://www.nseindia.com/content/press/Download_Real_Time_Tariff_Domestic_New.pdf
Still L3 is pretty steep
Edit: Correction regarding the blog. Yes, they have mentioned that their L4 data is converted to mkt depth 20 and then broadcasted to clients.
Anyway, didnt want to make many changes to the post, but it does make one wonder. A lot of questions to ask