Check the listing for details. Source Audio One Series C4 Synth. Condition: New. Listed at 269.00 USD. On the surface, the C4 features a simple 4-knob interface and a choice of six amazing guitar/bass synthesizer effects - just plug-in and play. It’s that easy. Six Preset Positions: Use the C4’s three-position toggle switch and two preset banks to save six easily accessible presets.