Functions as a guanine nucleotide exchange factor (GEF) that promotes the exchange of GDP to GTP, converting inactive GDP-bound small GTPases into their active GTP-bound form (PubMed:12628187, PubMed:16464467). Involved in regulation of adherens junction between cells (PubMed:12628187). Plays a role in cell migration (PubMed:20679435). {ECO:0000269|PubMed:12628187, ECO:0000269|PubMed:16464467, ECO:0000269|PubMed:20679435}. [Isoform 2]: Has a higher guanine nucleotide exchange factor activity compared to other isoforms. . This is the function of ENSG00000128512 (DOCK4, dedicator of cytokinesis 4).