I am wondering if with Audio interfaces such as the Scarlett 4i4 is it possible to plug a mic with +48V phantom power in line 1 and plug a guitar into line 2? I mean, if I activate the +48V power, I guess it will be activated for both of the two frontal channels, and I guess the guitar will not work in a 48V line, or it is something automatic that the Audio interface can manage?
Yes, you can do this. According to focusrite, "48V Phantom power is not sent to any 1/4" jack inputs, these are used for line or instrument inputs." So, plug the microphone into your interface with an XLR cable, and plug the guitar into your interface with a 1/4" instrument cable. It will work.