Which among the Pious Caliphs (Khulafa-e-Rashideen) died a natural death? 

1. Hazrat Abu Bakr (R.A.)
2. Hazrat Ali (R.A.)
3. Hazrat Usman (R.A.)
4. Hazrat Umar (R.A.)

Only Abu Bakr (R.A.) died naturally (August 23, 634 CE) from fever at 63 years old. The others were martyred: Umar (R.A.) by Piruz Nahavandi, Usman (R.A.) by rebels, and Ali (R.A.) by Ibn Muljam. His final days included appointing Umar (R.A.) as successor.