Li4 is an abbreviation for the weight 4 polylogarithm function, i.e. Li4 = PolyLog[4, #]&.
Li4
Li4 = PolyLog[4, #]&
Overview, Li2, Li3, SimplifyPolyLog.
Li4[x]
Li4(x)\text{Li}_4(x)Li4(x)
Li4 // StandardForm (*PolyLog[4, #1] &*)
D[Li4[x], x]
Li3(x)x\frac{\text{Li}_3(x)}{x}xLi3(x)
Integrate[Li3[x]/x, x]