Bromodomain-containing protein 4 (BRD4) is a cancer therapeutic target in ongoing clinical trials disrupting primarily BRD4-regulated transcription programs. The role of BRD4 in cancer has been attributed mainly to the abundant long isoform (BRD4-L). Here we show, by isoform-specific knockdown and endogenous protein detection, along with transgene expression, the less abundant BRD4 short isoform (BRD4-S) is oncogenic while BRD4-L is tumor-suppressive in breast cancer cell proliferation and migration, as well as mammary tumor formation and metastasis. Through integrated RNA-seq, genome-wide ChIP-seq, and CUT&RUN association profiling, we identify the Engrailed-1 (EN1) homeobox transcription factor as a key BRD4-S coregulator, particularly in triple-negative breast cancer. BRD4-S and EN1 comodulate the extracellular matrix (ECM)-associated matrisome network, including type II cystatin gene cluster, mucin 5, and cathepsin loci, via enhancer regulation of cancer-associated genes and pathways. Our work highlights the importance of targeted therapies for the oncogenic, but not tumor-suppressive, activity of BRD4.