Can I make a 5ch radio?

[CG Bob]

The switch that does nothing is actually the wireless trainer switch. It is covered in the instruction manual - page 6 of the online version.

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.

You could ask Hobbico/Tactic to convert the trainer switch to a fifth channel.