Reply to S4 live input?

Hey DJTT Members! Quick question for all of you!
I have an S4 and a synthesizers. I would like to route the synth through the s4 through one of it's rca lines. Is there anyway of doing this without getting any sort of delay/latency issues from converting the signal through the s4? or does it just run it through like a mixer?
Also, what about the microphone input? Does that do the same as well?
Finally, I have used the microphone as an input routed to deck c or d before, allowing me to ad fx to my voice. But, there was a delay. Anyway around this?

Thanks all!
-Brent Watters