Misleading title - they’re not doing this by applying 4% tax to the income of mere millionaires, they’re doing it by applying 4% to the incomes of those with annual incomes one million and over.
That’s a much, much wealthier cohort than the title implies.
Remember that this is a marginal income tax rate, i.e. it does not apply to the first $1 million of taxable annual income, only to the amount in excess of that threshold.