I can help with this one. The iPhone4 is pretty much dead flat (which is why some people call it boring). As far as neutrality goes - the iP4 would sit between the Studio V3 and X5. With the V3 being neutral with a tinge of brightness, and the X5 being neutral with a tinge of warmth.
Thanks for your comments, I agree with you 100%. I have an iPhone 5s and even though the sonic quality has improved it is still distant from the players we both own and is really boring flat, more like dead flat however with certain phones, it can feel more alive but never to the extent of say the X5. The only Apple product that still got me puzzled is the iPad 3rd gen sound sig, I don't know if that specific version contains a different Cirrus Logic batch DAC but I have liked it more with some types of music than the other players I have. In lower res of course and file formats it can understand. CL is getting a real bad rap these days.