Say we don't use a single 4.0 functionality, will a 3.1.3 compiled app with the 4.0 patch work on iOS4? I only have one device so I don't have a way of testing this but I'm sure someone here has done it. If someone here does have 4.0 installed on the iPhone and if anyone cares, I can send you a 3.1.3 compiled app so we can figure this out!
I've tried this out. I converted my 3gs phone, installed itunes 9.2, and put the Ios4 on it. None of my apps initially worked on it, loading screen, then died. I did the patch, put 3.1.3, runs like a charm on my iphone running os 4. Only thing so far i'm working is the videos, they play on os3.1.3, os4 they dont play. A few minor scuffs, like some tables lines. The rest all works.