Oh you mean, like just a proportionally scaled up screen, no change in ratio?
Precisely. Of course swapping to a different ratio is 'real' 4". But I think that the usability improvements in increasing from a 3.5" to a 4" by just putting extra vertical real estate are minimal. Unless they're planning to vertically stretch UI elements, they'll all be the same size, which means you're getting ~170 extra pixel rows of content. That's like, three lines of SMS. I don't care about three extra lines of SMS. Give me larger UI elements.
I could imagine people who watch lots of videos on their phone are excited, but I'm not one of those people. A screen increase by swapping ratios really seems it has all the benefits of theft over honest toil.