I like this one too. When I read it in my head I say "iPhone 4". Whenever I read i4 it's more of an "eye four" type deal. Plus, i4 reminds me of some old Mac or an Intel processor like below.I like iP4
I'll probably keep calling it the iPhone 4. You missed one on the Don't list: iPhone 4G.
i4 sounds too much like an intel processor