Maybe Indy follows the Star Trek curse, except reversed. Where the even numbered movies are not on Par with the odd ones. Crystal Skull could be Indy's "Final Frontier" and maybe we're due an "Undiscovered Country" next...
By the by... I have solved the riddle of the Star Trek curse! Every time Nicholas Meyer was writer and/or director... it was good! Every time he wasn't... they sucked. So... maybe have Meyer write/direct Indy 5?