The switch that does nothing is actually the wireless trainer switch. It is covered in the instruction manual - page 6 of the online version.
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The TTX404 Tx includes a built-in wireless trainer function – no trainer cable required! This trainer system connects a teacher’s Tactic Tx to a student’s Tactic Tx by wireless connection. Tactic’s wireless trainer function is not compatible with trainer systems in any other brand radios.
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You could ask Hobbico/Tactic to convert the trainer switch to a fifth channel.