If George (Lucas) is jerking his friends Steven and Harrison, by telling them, he's working hard on the Indy 5 story, - but he isn't - this also could be a reason to take to the bottle or to turn their attention to other movie projects....
"Come on George! Let them and us finally know if you're not motivated to do Indiana Jones 5!"
So this is, why there is no definitive message about that there will not be another Indiana Jones movie:
abcnews.go.com/Entertainment/video/harrison-ford-star-wars-han-solo
George Lucas sees also no future in dead Indiana Jones toys (games, ,...merchandizing), so he is just pretending there could be another Indy-movie. If people hope a little bit, that there will be another Indiana Jones movie, they will buy Indy-merchandizing.
So for me it's obviously, that meanwhile the horse is out the barn.