In general looks good.

I can't tell how easy it is to use the instrument in the way it was being used as it was before (ie usually just with 1 S21 trace), is it possible to display just that or do you have to display all 4 or none at all? What happens when you assign 4 traces? Does it take them all? Will it be just as fast if I want to use the instrument for only one like I was doing before?

Is it possible to add channel and sindex to the docstings as I can't tell what they are really?

I had some idea that it would be nice to have the sindex as a parameter which you set (and then influences the code executed in the get call) rather than having a parameter for each sindex because that has the potential to grow rather large if people want more than s21 22 etc. This method just seems somewhat less flexible (how to extend to adding external generator?). Then you could have differnt traces as different channels (when we finally have channels implemented in a meaningful way in qcodes) which to me seems to be a more faithful implementation of what the instrument is doing not to mention my allergy to having numbers in the parameter names. That said it will probably fulfill 99% of user needs and I may well be overthinking it and if I wanted it a different way I should have written it myself