Background <p>Non-small cell lung cancer (NSCLC) is a leading cause of cancer mortality. This study examines whether the deubiquitinase USP4 promotes NSCLC progression by stabilizing PAF1 and opposing the E3 ligase CNOT4.</p> Methods <p>USP4 and PAF1 expression were examined in NSCLC cell lines and paired tumor tissues by Western blotting and RT-qPCR. Functional studies included USP4 overexpression or silencing, proteasome inhibition, and rescue assays with PAF1. Co-immunoprecipitation and ubiquitination analyses were performed to confirm protein interaction and K48-linked ubiquitin modification. Cellular proliferation was assessed using CCK-8 assays, and the oncogenic role of USP4 in vivo was evaluated through A549 xenograft models co-expressing USP4 and/or CNOT4, followed by histological and Ki-67 analyses.</p> Results <p>USP4 and PAF1 were concomitantly upregulated in NSCLC cells and tissues, displaying a strong positive correlation. In addition, in-silico analysis of available NSCLC datasets showed that high USP4 or PAF1 expression was associated with poorer overall survival, whereas high CNOT4 expression was associated with better overall survival. USP4 enhanced PAF1 protein stability by directly binding and reducing its K48-linked polyubiquitination, independent of transcriptional regulation. Functionally, USP4 promoted NSCLC cell proliferation and rescued CNOT4-induced PAF1 degradation. In vivo, USP4 expression restored tumor growth and proliferation suppressed by CNOT4, confirming its antagonistic role in tumorigenesis.</p> Conclusion <p>USP4 stabilizes PAF1 by counteracting CNOT4-mediated ubiquitination and degradation, thereby promoting NSCLC progression.</p>