For more on trade offs, if requirements mean additional external circuitry is needed to handle the current, then with a separate voltage source for the LEDs, and a driver that be switched by the Arduinio, the circuit could go back to the original 4 LEDs in series with a single limiting resistor. Arduino turns on/off driver with 3.3 or 5 volts, driver turns on/off string of LEDs using something like 9 or 12 volts. Here 'driver' could be a FET. I did that to control a 12V incandescent light bulb, which needs a lot more power than a few LEDs.
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