Do I need another power supply to add 4 LEDs to Arduino Romeo Robot?
What is the current rating of the power supply you are currently using ?
A regular LED shouldn’t draw more than 20mA.
9 volt battery connected 4 LEDs with 110 ohm resistor in series, if use DPDT Switch to control both the robot and the LEDs circuit.
A 9V battery can typically supply 500mA for one hour before being drained.
A typical LED will have a forward voltage of 1.8-3.3V. Worst case scenario, if the forward voltage of the LED is 1.8V and being powered with a digital pin from the Romeo Controller (5V), one LED will draw about 29mA (15.45mA if the forward voltage is 3.3V).
You should also consider how much the motors and controller are consuming but you shouldn’t have an issue powering 4 LEDs with your battery.