The tumor-promoting arm of transforming growth factor beta (TGF-β) receptor signaling contributes to advanced cancer progression and is considered a master regulator of breast cancer metastasis. In mammals, there are six distinct members in the tumor-necrosis factor receptor (TNFR)-associated factor (TRAF) family (TRAF1-TRAF6), with the function of TRAF4 not being extensively studied in the past decade. Although numerous studies have suggested that there is elevated TRAF4 expression in human cancer, it is still unknown in which oncogenic pathway TRAF4 is mainly implicated. This review highlights TGF-β-induced SMAD-dependent signaling and non-SMAD signaling as the major pathways regulated by TRAF4 involved in breast cancer metastasis.