Output 4 is what you want- meaning a cable that will plug into ‘4’ (output) going into your computer, which 99% is gunna be 3.5mm.
Looks like the out from your keyboard is 1/4” stereo (two black lines on that side of the plug) <——-> 3.5mm ‘in’ to computer.
Something like this probs?
https://www.amazon.com/UGREEN-Housing-Compatible-Theater-Amplifiers/dp/B00Y2LANUU
RCA outs/Ins are for like record players and turntables. you needs a Converter or something to use that.
you need to 1/4in to 1/8in audio cable and a 1/4 Male to female splitter - along with the iPhone audio adapter - maybe not these specifically, I would buy the cables in person, but these types.