In a nutshell, it has to be seen.
The PoE out of the cap Ax is:
- passive
- mode B
In theory a 802.3af compliant device (PD or Powered Device) should be compatible with both Mode A and Mode B.
See:
http://forum.mikrotik.com/t/hex-poe-48v-802-3af-at-only-b/127845/1
The 4 unused cable are not unused in a 1 GB connection anyway.
The device providing power (PSE or Power Source Equipment) can then be either mode A or mode B, the difference with passive is that the 802.3a compliant device “interrogates” the other end by “probing” it with - initially - a very low voltage and expects a reply before switching on the 48V or so power, so called PoE negotiation.
The (passive) Mikrotik has only a simpler check for resistance on the other side if auto-on:
https://help.mikrotik.com/docs/display/ROS/PoE-Out
which may (or may not) be enough in the PD to switch on the power.
You can still force it on with forced-on.
Then the PD may either “accept” the 48V or so volts or - because there has not been a previous PoE negotiation - I believe that different devices may have different implementations.
In some cases a splitter/converter and powering the device from the DC jack may be needed.
Do you need/want a 10/100 or a 1000 Mb connection between the two devices? ← if needed, splitting the power off a Mode B 10/100 connection can be a DIY job besides using common splitters (otherwise a GB splitter is needed)
Can you post the exact make/model of the powered device? ← Maybe some member have already some experience with the specific device