That is the Douglas DC-6 / C-118/R6D Liftmaster.
This specific plane was built in June 1947 with modifications for presidential use. It is well known for flying US President Truman. It was the first presidential aircraft that received a distinctive exterior.
It actively served USAF until 1965 when it was transfered to an USAF museum. It is now on display at USAF museum in Dayton, Ohio, USA.