Most families use all four and rarely decide the proportions on purpose. The order they are drawn on changes the total cost of the degree considerably.
The four year degree is treated as the safe choice and the thing to do while undecided. It is a large commitment and it deserves to be chosen rather than defaulted into.
The offer refused in month two is often better than the outcome reached in month fourteen, once somebody counts what the intervening year actually took out of them.
There is no single rulebook for state and local obligations and nobody mails one out. That is why the first notice usually arrives from a state you barely worked in.