A novel ErbB-4 ligand, referred to herein as Neuregulin-4 (NRG-4) is disclosed as well as polynucleotide sequences encoding NRG-4, oligonucleotides and oligonucleotide analogs derived from polynucleotide sequences, a display library displaying short peptides derived from NRG-4, antibodies recognizing NRG-4, peptides or peptide analogs derived from NRG-4, and pharmaceutical compositions and methods of employing peptides or peptide analogs, oligonucleotides and oligonucleotide analogs, and/or polynucleotide sequences to up-regulate or down-regulate ErbB-4 receptor activity (signaling).