First, 3* mats (midnight roots) are slightly different than 4* mats (orichalcum nuggets)
Second there are levels that are best for any specific mat, including 3* and 4* mats … and there are levels that have the most total 3* and 4* mats (which happen to be the same)
Personally I find it more productive to farm for general 3* or 4* mats than a specific one, at least most of the time, and especially during Atlantis Rises. Your situation may differ.