You may be correct, but I don’t know if we know that yet, for three reasons:
1) iOS and PS for iOS might employ agressive forms of memory compression. A huge PSD file of 3GB (like the one in the Apple demo) may use less than 3GB of RAM when open due to memory compression.
2) The files for the iOS version of PS are not standard PSD files. They’re a type of PSD file optimized for Creative Cloud. Not sure what this means exactly, but could imply a difference in size and how they are handled.
3) I was listening to an interview on the Verge podcast with one of the leaders for the Adobe PS team. He said something to the effect that you would be able to edit your projects even on a plane with no internet connection because thumbnails (not sure if that was the term, but some type of small file) are stored locally that allow you to edit, then when you have a connection your edits will be sync’d with the cloud. This implies you will not need the entire full PSD file open in order to make edits and do work.
Again, you may be correct that 4GB of RAM just won’t be enough, but I think there are reasons to say, “wait and see”.
No. 3 sounds like how Mylio (my photo management and photo editing software) works on my iPad. My library of originals is 59 GB; my library of previews on my iPad is 5 GB. I can however edit these preview images on my iPad just as I would on the desktop version, and without an internet connection, and these edits are then automatically synced to my originals when I have a connection.