Arduino Motor Shield

Can I Control 1 4V Servo And 2 DC Motors. The Robot Will Use Ultrasonic Sensor, Stop At An Obstale, Use Servo To Scan And Then Move On.

Thank You

Yes. Likely want a seperate supply for the motors, can be connected in parallel to the barrel jack connector.
Servo needs a control signal, motors may need Arduino to control transistors, or an H-bridge, to drive the motors.

How Do I hook Up An External Power Supply?

Thank you In Advance,

Make up a cable that connects to the source, and has 3-4 leads in parallel - one to the Ardiuno, the servo, and the 2 motors.