Answer: FDR. Franklin Delano Roosevelt.
Step-by-step explanation:
Franklin Delano Roosevelt is the only President who had served 4 terms.
Since 1 term is equal to 4 years, he was supposed to serve the country for a total of 16 years.
Unfortunately, during his 4th term he had died from a cerebral hemorrhage, abruptly ending his Presidential career from 1933-1945.
The reason why FDR was able to serve so many terms was because that being elected two terms was only tradition and there was nothing at the time preventing an individual from serving more than the traditional amount.
FDR's death had such a grand impact that in 1951, the constitution ratified the 22nd amendment; officially making 2 term limits the max limit a President can serve.