Avoid the M4
I can't update iPlayer due to the OS taking up 5GB of the phone's 8GB total. Sony have put 2GB of unremoveable crap on it as well. The M4 cannot be rooted.
The phone is crippled to a state of uselessness the moment you take it out of the box.
The shiny transparent plastic back can be scratched by placing it on a hard surface. A case is essential.
The camera is catastrophically bad.