its for the month and day the model released in 2026. 0813 -> Aug 13th. I assume that if they use it internally the padded 0 makes finding the newest model easier cause all the numbers for the date line up instead of the zig zag you get without it once you get to 2 digit months.
Wondering, sorry if it's a dumb triviality to ask.
Is this even a (sub-)version number? I mean the major version is clearly 4.