To me, it is really superfluous to do a bigger screen but struggle on the battery life front. I would rather they keep it at 4 inches till battery life gets to the point of exceptional, while figuring out how to get that optimal mix of screen resolution and battery life for a device below 5 inches but above 4 inches.
No point going above five inches if they aren't really progressing below that...I don't really want to see an iPhone and iPad basically looking alike...
With that in mind, it's harder to make a device like that while staying true to the usual iPhone design structures.