The term POLIO SCIENTIST refers to Jonas Salk, an American medical researcher and virologist who developed the first successful inactivated polio vaccine. He is best known for his work in the 1950s when he led a team of researchers in the development of the vaccine, which was declared safe and effective in 1955. Salk's vaccine was made available to the public in the United States in April of that year, and it quickly became the standard for polio prevention. The four-letter solution to the crossword is SALK.