1935: <i>Temple of Doom</i>
1936: <i>Raiders of the Lost Ark</i>
1938(approx): <i>Last Crusade</i>
1957: <i>Kingdom of the Crystal Skull</i>

It's also rumored that Shia LaBeouf will take up the "mantel" of the whip and Harrison Ford will make an appearance later in the fifth movie as a mentor much as Sean Connery did in <i>The Last Crusade</i>.