Same thing happened with me.....
My 3rd Main out never worked and had to replace it with 5th one using "Channel Output Function" settings .....Don't know why it happened but maybe because of cheap clone pixhawk.
Here's the link to get more details on it (every settings of pixhawk can be changed from here) , I would suggest to read it carefully....
https://ardupilot.org/copter/docs/parameters.html#mot-pwm-typeI don't remember what exactly I did back then but something like this......
Here's the default values of the channel output function...
33 Motor1
34 Motor2
35 Motor3
36 Motor4
37 Motor5
38 Motor6
39 Motor7
40 Motor8
Just by assigning different value to servo*_function, we can change the layout
In your case, what you can do is replace 5th one as 4th motor or 4th main out and so on 6th, 7th (onward 6th main out only if you are trying hexacopter, octa, etc)
You need to assign "
SERVO5_FUNCTION to
36" - This will assign 5th main out to motor 4 & assign "
SERVO4_FUNCTION: to anything and don't use it"
A word of caution, you should change these settings only when you know what you are doing & ground test everything 1st without prop !!