Mikey Digital includes a guitar adapter for connecting instruments such as guitar and bass to the multi-source 3.5mm inputjack, which also accepts stereo line-in and mic-in.
The back of the S4 is pretty much what you'd expect: a couple of spare sets of RCA stereo inputs, a micinput, TRS and RCA outputs, a MIDI passthrough, and a footswitch jack if you're into that sort of thing.