如何使用“四重嵌套”将宽数据帧转换为多级结构的长数据帧？

I conducted a study that, in retrospect (one lives, one learns :-)) appears to generate multilevel data. Now I'm trying to restructure the dataset from wide to long so that I can analyse it using e.g. lme4.I conducted a study that, in retrospect (one li