Although transcriptomic studies have stratified pancreatic ductal adenocarcinoma (PDAC) into clinically relevant subtypes, classical or basal-like, further research is needed to identify the transcriptional regulators of each subtype. Previous studies identified HNF4α as a key regulator of the classical subtype. Still, the distinct contributions of its isoforms (P1 and P2), which display dichotomous functions in normal development and gastrointestinal malignancies, remain unexplored. Here, we show that HNF4α-positive human PDAC tumors exhibit uniform expression of P2-isoforms but variable expression of P1 isoforms. To dissect the roles of each isoform in PDAC, we performed functional, transcriptomic, and epigenetic analysis after exogenous expression in HNF4α-negative models or CRISPRi-mediated knockdown of endogenous isoforms. We demonstrated that P1 isoforms are less compatible with growth and stronger transcriptional regulators than P2. Despite both isoforms sharing a common DNA-binding domain, P1 isoforms displayed stronger binding at HNF4α target genes, resulting in increased transcriptional activity. These findings provide a detailed characterization of HNF4α P1 and P2 isoforms and their distinct roles in PDAC biology. Implications: HNF4α isoforms exhibit heterogeneous expression in PDAC and have distinct effects on proliferation and gene expression, including markers of clinically relevant molecular subtypes.