Ah. Here's the tool for that job - a Roper-Whitney hand punch.[1]
This is like a hand paper punch, but stronger. Costs $85.
Made in USA.
With a punch and die, you get clean edges on the hole on the exit side.
Scroll down for how to order the specific punch and die you need. Hex punches are not common, but are available. If you fill out their form, and tell them you need to punch 3CR13 stainless, they'll tell you what to order. Might need something with more leverage than the small hand punch. Lubrication helps.
Most machine shops will have such a punch, but they won't have a 4mm hex die in stock.
Or you could drill an undersized round hole and file it out to a hex hole. Maybe use the existing round carabiner hole.
There are touch-up kits for guns, called "gun bluing", to make your shiny new hole black.
This is all do-able but way more trouble than it is worth.
Most machine shops will have such a punch, but they won't have a 4mm hex die in stock.
Or you could drill an undersized round hole and file it out to a hex hole. Maybe use the existing round carabiner hole.
There are touch-up kits for guns, called "gun bluing", to make your shiny new hole black.
This is all do-able but way more trouble than it is worth.
[1] https://www.roperwhitney.com/our-products/no-5-jr-hand-punch...