The astronauts will be put through many physical checks before they leave. At some point however, an astronaut on Mars will get ill or will get involved in an accident. Medical equipment will be present on Mars and on the way to Mars to treat the most common injuries and illnesses. Two of the four astronauts will have received comprehensive medical training, and the other two will have extensive knowledge of first-aid. All these elements together will provide the group with the tools to help itself.
The medical possibilities on Mars will be more limited than those of a modern hospital. Big, heavy equipment won't be present in the settlement for the first few years. Certain conditions will be more difficult or even impossible to treat on Mars.
Subsequent years will see more advanced medical technology make its way to Mars to make more complex care possible.