the same number have died in office of natural causes (4), and depending on how you want to look at it, many more probably died prematurely (within a few years) after leaving office, likely do to the stresses involved in the job (wilson and lbj surely qualify)
Bill Clinton too. Man, that guy is aging fast. Hillary still looks pretty good for her age. I wonder what 4, or even eight years at the helm would do to her. I guess we'll never know.
My (least) favorite "died in office" is Harrison. What kind of arrogant fuck gives a four-hour long inauguration speech in a downpour?
Look at the "before" and "after" photo of just about any recent president and the profound effect of the office is obvious. They age way more than the 4 or 8 years in office. Carter, Clinton, and W all come to mind.
Reagan looked pretty much the same but he was old to begin with and dyed his hair.