Multiple definition of "..." Compiler Error
Although the motor (global) variables are defined in a header files, they are included in both cpp files, thus defined twice.

Make them extern, and than define them in the motor.cpp file.

In motor.h, change

Servo motor1, motor2, motor3, motor4;
Servo motorArray[4] = {motor1, motor2, motor3, motor4};


to

extern Servo motor1, motor2, motor3, motor4;
extern Servo motorArray[4];


In motor.cpp, add:

Servo motor1, motor2, motor3, motor4;
Servo motorArray[4] = {motor1, motor2, motor3, motor4};


As a side note, use consts for values 4 and 2000.