3-4 inches are very young, not like the 20" adults.
In the picture, look at the size of that filter. It's huge. That's what you need if you wish to overstuff your pond.
The problem, though, is that filters break down sometimes. You want to keep you stocking level, in ALL tanks and ponds, in a range that would be safe even despite a very longterm loss of your lifesupport equipment.
Also look at how clear that water is. That's not normal. The owner of this pond is doing something else to help his fish make it in this pool. UV radiation and many frequent large waterchanges are my guess, and these can certainly help keep koi happy in crowded conditions. ( at least until something goes wrong, of course )
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