What other field can one specialize into after four years of medical school?

After four years of medical school (resulting in an MD or DO degree), a physician doesn't specialize *into* a field. Instead, they then pursue residency training, which is postgraduate medical training lasting anywhere from 3-7 years (or even longer for some subspecialties) to specialize in a specific medical field.

So, the answer isn't specializing *into* a field after medical school, but rather choosing a specialty to pursue *during* residency. Examples of specialties include:

* Internal Medicine: Focuses on the diagnosis and treatment of adult diseases. This can then lead to subspecialties like cardiology, gastroenterology, nephrology, endocrinology, etc.

* Surgery: Includes many subspecialties like general surgery, neurosurgery, cardiovascular surgery, orthopedic surgery, etc.

* Pediatrics: Focuses on the care of children.

* Obstetrics and Gynecology (OB/GYN): Focuses on women's health.

* Psychiatry: Focuses on mental health.

* Family Medicine: Provides comprehensive care for individuals and families of all ages.

* Emergency Medicine: Focuses on acute and critical care.

* Radiology: Involves interpreting medical images.

* Pathology: Involves diagnosing disease through laboratory testing.

* Anesthesiology: Focuses on pain management and the administration of anesthesia during surgery and other procedures.

And many more. The choice of specialty depends on the individual physician's interests, skills, and career goals.