Stereo channels with 2 unbalanced stereo inputs on each-one RCA and one TRS jack.

路立体声通道，每路均具有2 个非平衡式立体声输入，一个是RCA 插口，一个是TRS 插口。

youdao

So, it's out with the old 30-pin dock, but there's still optical, AUX, video out, RCA and 3.5mm jack connections if you prefer to keep things physically connected.

ENGADGET

The back of the S4 is pretty much what you'd expect: a couple of spare sets of RCA stereo inputs, a mic input, TRS and RCA outputs, a MIDI passthrough, and a footswitch jack if you're into that sort of thing.

ENGADGET: Native Instruments Traktor Kontrol S4 review Alt