Only Input Channel 1 and Input Channel 2 are compatible with the signal chain in the Spark app to deliver built-in effects.
Input Channels 3 and 4 do not support built-in effects from the Spark app.
For Input Channel 1, the Spark app can fully control and save the guitar signal chain.
For Input Channel 2, the Spark app can manage certain aspects of the signal chain. Additionally, you have the option to add preamps and effect pedals specifically designed for acoustic guitar, bass, and microphone.
See also:
Adjust the signal chain between Input 1 & Input 2