The fourth president of the United States of America is James Madison. He served 2 full 4-year terms, which means he served for eight full years straight, from 1809-1817. He was born on March 16, 1751, and he died on June 28, 1836. He also made a major contribution to the ratification and approval of the US Constitution by writing up "The Federalist Papers," as well as John Jay and Alexander Hamilton. He was later referred to as the "Father of the US Constitution," during the later years of his lifetime. Anyways, to answer your question, the fourth President of the United States of America was James Madison. Hope this helped answer your question. Good day to you.