What happens if you’re sure that four decimal places is the smallest, then suddenly a partner system starts sending you 6 decimal places?
e.g. it doesn't make sense to support billing in sub-currency unit amounts just by allowing it in your API definition, as you're going to need to batch that until you get a billable amount which is larger than the fee for issuing a bill. Even for something like $100,000.1234, the bank doesn't let you do a transfer for 0.34c.
For cases where sub-currency unit billing is a thing, it should be agreed what the minimal unit is (e.g. advertising has largely standardised on millicents)
https://en.wikipedia.org/wiki/ISO_4217
Or choose a different standard, I don’t know what else is out there, but you probably should choose an existing one.
If you're doing ads and going for millicents, something like $0.01234 should be encoded as 1234 millicents.
Obviously you have to agree on what you're measuring in the API, you can't have some values be millicents and others cents.