The author proved the following main result: Let \(f\) and \(g\) be meromorphic functions sharing four small functions \(a_1,a_2,a_3,a_4\) ignoring multiplicities. If there is a small function \(a_5\) distinct from \(a_j\), \(j=1,2,3,4\), such that \[ \overline{N}(r,f=a_5=g)\not=o(T(r,f))\;(r\to\infty) \] possibly outside a set of \(r\) of finite linear measure, then \(f=g\), where \(T(r,f)\) is the Nevanlinna's characteristic function of \(f\), and \(\overline{N}(r,f=a_5=g)\) is the valence function of common zeros of \(f-a_5\) and \(g-a_5\) counted only once (ignoring multiplicities). Particularly, if \(f\) and \(g\) share \(a_5\) ignoring multiplicities, this result was proved by \textit{Y. H. Li} and \textit{J. Y. Qiao}, On the uniqueness of meromorphic functions concerning small functions, Sci. China, Ser. A 43, No. 6, 581--590 (2000; Zbl 0970.30018)].