This statement goes along with an old oriental saying that says: “Choose a job you love, and you will never have to work a day in your life”. Let’s say you are a doctor, who, like you said, loves being a doctor and help and save lives. If this person truly loves his job, then he/she could dedicate 4 hours a day (or 20 hours a week) to earn his/her own income that will allow a comfortable life and then probably dedicate 2 or 4 hours offering medical care that do not have enough income to afford it.
We could also take a lawyer as an example, that could offer legal assistance to the poor, or an engineer that could help build houses for people in need. There will not be any restriction on how someone spends their leisure time, they could even turn it into paid time, as it was explained in a different question, but if such person really loves what they are doing, there are plenty of ways to channel that desire that could benefit him/her self or society as a whole. They could even dedicate their leisure time to start their own business without quitting their job and leaving it as a main source of income. Reducing the work-day to 4 hours would create then the right environment for entrepreneurs and reduce their risks of loss of income during the transition, since they will have both a main source of income and extra time to develop their idea.