x1 (li) is an x3rd root of x2, with other (identifying) characteristics x4.
This word is related to seltenfa in basically the same way that dugri is related to se'ai'e zei tenfa. Notice that each of fenfa, tenfa, and dugri each can essentially be expressed by permuting the terbri of any one of them. Thus, this word fixes an asymmetry in the definitions. Since there can be many nth roots of a radicand, x4 allows for a means of uniquely describing/identifying which one of these options x1 is. See also: fe'a.