I had some spare time on my hands so I made a quick excel sheet that I can load up later. But the answer until next year will be 1861 and 1862.

There were 5 former Presidents alive who were :

van Buren March 4, 1841 - July 24, 1862

Tyler March 4, 1845 - January 18, 1862

Fillmore March 4, 1853 - March 8, 1874

Pierce March 4, 1857 - October 8, 1869

Buchanan March 4, 1861 - June 1, 1868

There have been, at times, 4 former Presidents alive at one time:

1825-26 (Adams, Jefferson, Madison, Monroe)

1845 (Quincey Adams, Jackson, van Buren, Tyler)

1993-94 (Nixon, Ford, Carter, HW Bush)

2001-2006 (Ford, Carter, HW Bush, Clinton)

Assuming that Carter, HW Bush, Clinton, GW Bush, and Obama all survive until Inauguration Day, there will be 5 ex-Presidents again.