Yeah I don't understand this one. If you're going to give someone 4 weeks off (which is extremely generous in the US) just give it to them at the beginning of the year.
Some places let you take the vacation before you accrue it but if you were to quit before earning enough to cover the days you've taken you owe the company money.
4 weeks off is not necessarily "extremely generous." It depends on whether that includes holidays.
You might say that of course vacation days don't include holidays, but I know of several small tech/biotech companies that just give X total days of paid time off, usually 25-30. You can choose to take those PTO hours on holidays or not, which is more flexible I suppose.
However, depending on the state and industry, there are 10-15 normal holidays every year. So, that 4 weeks off might really only equal 1-2 weeks of vacation, not so generous.
I get 20 actual vacation days. I think maybe 8 recognized holidays and 2 personal choice holidays and I think another 2 holidays that are chosen by work site. So I get 30-32 days off per year which is pretty generous. For most people I know you would need to work 15 years before you earn even 4 weeks and that is usually the max amount.
I'd say it's common for most white collar jobs to give you 2 weeks off until you hit 5 years then you might earn another week.
Some places let you take the vacation before you accrue it but if you were to quit before earning enough to cover the days you've taken you owe the company money.