Hi,
the tank is definitely too small. Koi don't spawn with a size of 4" or you have dwarf kois. Koi spawn at the age of 2 till 3 years and normally they should have a size of 1 until 1,5 feet at least. A pond with several cubic meters of water is the best thing for them because they are really large fishes and they like to spawn among the plants in shallow parts of the pond. Let them find their partner theirselves and watch how wild they can get during the mating.
regards
Robert
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