This is mostly for posterity since I couldn't find the answer anywhere else pre-purchase. My Gen 4 Rail already had a wire installed for a front light. It was tucked into the frame under the top tube controller cover.
I can confirm that Gen 5 Rails are not pre-wired for a front light. You'll have to run a cable from the motor yourself.
I can confirm that Gen 5 Rails are not pre-wired for a front light. You'll have to run a cable from the motor yourself.