Indiana Jones 4 isn't official yet, there's just been a lot of rumors. Contracts haven't been signed, and Lucas Entertainment doesn't even have a plot yet. All that's been done is the hinting of an Indiana Jones 4, and Ford has hinted like there's no tomorrow that he wants a whole slew of money. Also, if there is going to be another movie, Sean Connery has stated that he would like to contribute. Ha! If you think Harrison Ford will be old, think of Connery!
Anyways, I can't think of anyone else to play Indiana other than Harrison Ford. To see anyone else put on that fedora would just feel so...dirty...