Partial arming failure, motor number 4 refuses to respond

I need help. The problem I have encountered is motors 1, 2 and 3 spin up when I arm my quad but motor 4 is dead. The quad is a HobbyKing Dart 450. Now for the technical bit, I use QGroundControl Version 3.5.0 I have flashed the Flight Controller (Pixhawk 2.4.8.) with ArduCopter 3.6.6. To condense 2 weeks of fiddling and web searches I have tested motor 4 and when plugged into Motor 1 position on the FC it functions so I assume the problem is internal. Further I assume somewhere in the myriad of parameters that one figure is wrong to cause this but I am not familiar enough with the parameter list to know where to even begin. I have selected the X copter as the vehicle and followed the setup screens as I have done with my previous 3 quads fitted with Pixhawk controllers. I would appreciate some help, thanks in advance.

I have solved the problem, just a real bad on my part, count the pins more than twice before plugging the ESCs in.

Don’t worry everyone has done that.

Thanks Mike I need all the help that people are willing to offer but a bit afraid to ask.