Because it is now a rule, we tend to forget that some time ago Presidents of the United States could run for the office as many times as they wanted to. Most Presidents stuck to a 2-term limit as was established by George Washington, but this was not an official rule but more of a courtesy.
source
Franklin Delano Roosevelt, mor commonly known as FDR, presumably to save on printer ink is a bit of a legend as Presidents are concerned because of many reasons. He was elected president during The Great Depression and his policies are seen largely as reasons why many humanitarian crises were mitigated and that the USA recovered from it. He also was president during World War 2 and is generally regarded as handling that little situation pretty well. One of the most remarkable things (to me) is that he spent a majority of his political life confined to a wheelchair due to losing the use of his legs because of polio.
source
While he was generally regarded as having done a fantastic job as president, it was his 4 terms in office that inspired the 22nd Amendment to the Constitution which dictates that no one person can serve as President for longer than 2 terms. Believe it or not, there were some influential people (including James Madison and Alexander Hamilton) who wanted the office of the President to be a lifetime appointment. Imagine the crying of lefties today if that was the case.
There never was before, nor will there ever be again, a President who serves more than 2 terms and on the 11th of July, 1944, FDR publically declared that he would run for a 4th term.... and he would later win comfortably gathering 432 out of the 531 electoral votes.
source