The first landmark in medical history among the given options is plastic surgery.

Option 1 is incorrect because organ transplants are a relatively recent development in medical science. The first successful organ transplant occurred in the 1950s.

Option 2 is wrong because bypass surgery was also not introduced until the mid-to-late 20th century.

Option 3 - the birth of a test-tube baby, was first achieved in 1978, hence making it a later milestone.

Option 4 is correct because the practice of plastic surgery has ancient roots, with written records dating back to ancient India around 800 BC. This form of surgery was mainly used to reconstruct noses, ears, and other body parts which were amputated as criminal punishment. Hence, plastic surgery is the earliest medical milestone among the ones provided.