Damn. Less than 4 weeks/year would be illegal here (it’s also mandatory that an employee can take at least 2 weeks of that as a continuous single block, if they wish.) Employers are also not permitted to buy holiday allowance back (except when an employee leaves) and must roll any unused over year on year - which means employers are strongly motivated to have their staff actually take their holidays.
“Here” is, obviously, not the US…
As an employer (and thus an evil capitalist pigdog etc.) I’ve always applied the rule of the thumb that “twice the holiday duration as notice” is polite to the employer and your colleagues who will cover for you - so two days notice for a day off, two weeks notice for a week off, a month’s notice for two weeks off, etc.
“Polite” does not mean “mandatory” of course - circumstances can dictate that sometimes you have to break the rule of thumb and that’s fine.