Something like Roland UM-One MK2. For the FI 4, any generic USB-to-MIDI cable should work (though I’ve not tried to see if my phone would handle such a cable yet). If using an iPhone then the Apple Camera Connection Kit adapter is also needed.
In terms of apps, the free version of TouchOSC, Midi Designer Pro X or any of the other free MIDI controller apps which can send CC and PC messages will work.