I am running a 13X6.5 with 5s on my Monster power 46, a copy of the Eflite. I too am using a 60 amp ESC. So the 12X8 should be fine on a 4s.
Here is the calc program I used to see if I would melt anything.
http://www.s4a.ch/eflight/motorcalc_e.htm
I have since used a watt meter just to make sure I won't melt anything or let the magic smoke out.
Buzz.