How to properly run 4 DC motors with an Arduino?
With out knowing the demands of the motors (how much current they need under a given load) it is difficult to answer your question.  But, in most Arduino designs: 


Motors are powered independently of the Arduino.  And when Arduino Uno is powered through its USB port it should be realized that most common USB power sources are only obliged to offer up to 500mA of current.   
The motor's power source likely needs to be capable of providing several Amps. A common 9 Volt battery only contains about 500mAh and will likely drain in 10s of minutes if it works at all.


Consider the power pack used in common portable electric power tools.  This should give you a perspective of the type of power you will need in order to create a robot with multiple motors.