They've been definately doing it for at least ten years now. The biggest obstacle is attempting to get the egos of Lucas and Spielberg in line, since one or the other has repeatedly vetoed assorted script ideas and production directions over the years.
The original basic plot of the story, of Jones looking for either Noah's Ark or the Garden of Eden, have long since been abandoned. Ideas as stupid as a quest for Atlantis have been floated, as well as casting Tom Selleck as Montana Jones, Indy's evil brother (someone kill whomever came up with that one, please. Slowly, please. Ever so slowly.)
News on the perennially pre-production
Indiana Jones 4 can be found at
The Raider,
KillerMovies.com, and
Coming Soon.