On the other hand, if you get the right experience, you don't necessarily need 4 years to learn it. During those 4 years, you probably spent a lot of your time doing the same tasks repeatedly, many of which did not require much knowledge or skill, and perhaps you learnt a new thing every day.
Not everyone can learn 10 new things a day, or gets the opportunity to, but you will appreciate there is a world of difference between a situation in which you are trying to get a job done and earn a wage, while learning as you go, and a situation in which the person you are working with is actively trying to get you to learn as much as possible.
My point being that 10 days is far too short a time, but a fast learner, in the right conditions and with the right people (a situation that a training centre could, hypothetically, bring about), could learn the trade in significantly less than 4 years. Which doesn't mean this is what is happening in practice.