We already know M4 is the codename behind the HTC One mini. The handset is supposed to be a mini version of the 4.7" HTC One (M7) flagship powered by a dual-core chipset and running on a smaller 4.3" 720p display.
Today, the HTC One mini was certified by the Bluetooth SIG, so if the live pictures weren't enough to convince you of its existence, the certification will do the job. The document also points to the fact that the launch of the device is drawing close and we should see an announcement sooner rather than later.
In case you wonder how we know HTC PO58XXX means HTC M4 - the M4 already appeared under the same model in the Wi-Fi certification database.