It works fine. Just plug in the output master from the NS7 via RCA to the back of the S4. Choose either channel C or D, make sure that it is switched to line. From the software preferences you make the necessary audio routing.
thereās a place we play with a similar setup - iām connecting the other guyās master out to the Channel āDā on the S4 in āTHRUā mode (the small swtich on the back). Thereās no need to mess with Live inputs or so - itās just mixing it to the master through an analogue chain.
the bad is that you wonāt be able to leave the party with your S4 before he is done. On the good side, you can happily switchover back-and-forth all night long