As bad as this sounds, everytime I hear about this my immediate reaction is "does this really have to be willed into existence?"
Obviously John has a lot of pressure under him -- you don't break up a certainly finite nature of a trilogy just for a movie that didn't demand it. But after two year-long delays, my set of mind is currently that if it's really this hard and difficult, why bother?
I'm not saying to completely scrap it, but if they downgraded it to a Valentine's Day TV special (even an hour-long one), the sting and the stakes would be significantly removed. Currently there's a problem and it's probably perfectionism. It's just too much for this story, IMO. TS4 could be great, granted, I'm just not sure if it's really worth all of this distress.
A good comparison is Indy 4 (Crystal Skull)... which, on its own, is really just an okay/mediocre adventure film with a few fun setpieces. But due to the immense pressure it was under, it created an enormous reaction of pure letdown for people, leading to its probably unwarranted mass hatred. This was especially true due to the fact that Last Crusade was seen as a perfect sendoff for the character, now sullied by this fourth film.
I hope to be proven wrong but if they let it go in any number of forms, I would not be letdown either.