Here's what you can do to your current setup.

In my opinion, sound quality and volume gain control will be much better on external mixer as compared to the S4. I highly recommend this kind of setup to be integrated into a 4 channel mixer like DJM800.

*S4 as controller for deck A & B. All the keys, jog wheels, filters and FXs will remain working except for the faders and cross faders.

*SL3 assigned as sound card for deck A & B to be connected to djm400 mixer. Can connect as many channels depending on your sound card n mixer.