if you hav full attendence for games, and your lvl 4 PR still give you low income, you can get off with a lvl 2
you can even pick a specialty and pay less
i upgraded my PR to lvl4 this season because i wanted more people to come to my games, and better merchandising.
actualy, it worked pretty good.
but if your PR doesnt worth the extra sallary you can employ a cheaper one.
BTW, i still use a lvl2 doctor, and suffered no injuries for the past 2 seasons :)