Create Where-Used Report Hierarchially

I don't feel that this is what you want, however perhaps it will be of some help.
Columns H:I display level 2 parts that are used in level 1.
Column H is populated using: =UNIQUE('Level 1'!B2:B637)
Column I is populated using: =TEXTJOIN(",",,UNIQUE(FILTER('Level 1'!A2:A637,('Level 1'!B2:B637=H2))))
Similarly columns K:L display level 3 parts that are used in level 2 and columns N:O display level 4 parts that are used in level 3.