There is a way, actually several... ;-) For a better understanding of how it's handled from the mixer: You're not routing a channel TO another channel - you choose where a channel gets its signal FROM! Channel 3 (analog input source) is guitar amp, so channel 4 should be the "copy".
Select channel 4, go to Settings -> Routing -> Change XLR symbol to "3". That's it. Now channel 4 sources its input signal from channel 3.
Use the input delay (7 to 10 mS) on channel 4 to "widen" the sound. It may be necessary to increase volume of channel 4 a little bit. Use the "Make-up Gain" of the compressor for this task.