Obesity exacerbates breast cancer metastasis, yet the underlying mechanisms remain incompletely understood. Here, we identify neuregulin 4 (NRG4), a ligand of Erb-B2 receptor tyrosine kinase 4 (ERBB4), as a key regulator of metastasis, through the ERBB4-YAP1 signaling axis. Using MMTV-PyMT and 4T1 breast cancer models, we demonstrate that obesity accelerates metastasis, while NRG4, secreted by inguinal white adipose tissue (iWAT), inhibits cancer cell migration and epithelial–mesenchymal transition (EMT). Mechanistically, NRG4 activates ERBB4, producing a cleaved pERBB4 fragment that interacts with phosphorylated YAP1 (pYAP1), restricting its nuclear translocation. RNA sequencing revealed that NRG4 suppressed the transcription of Mmp9 and Mmp12, which encode matrix metalloproteinases critical for extracellular matrix remodeling and invasion. Co- immunoprecipitation and promoter assay confirmed that YAP1 bound to TEAD1 and activated MMP9/MMP12 transcription in the absence of NRG4. Importantly, recombinant NRG4 (rNRG4) reduced the growth and invasiveness of breast cancer organoids. These findings establish NRG4 as a metastasis suppressor in obesity-associated breast cancer by inhibiting the ERBB4-YAP1 pathway and down-regulating matrix metalloproteinases. Our study highlights the therapeutic potential of targeting NRG4-ERBB4 signaling to mitigate obesity-driven breast cancer progression.