Auxin response factors (ARFs) as transcription activators or repressors have important roles in plant growth and development, but knowledge about the functions of wheat ARF members is limited. We identified a novel ARF member in wheat (Triticum aestivum), TaARF4, and demonstrated it was constitutively expressed. Subcellular localization indicated TaARF4-A protein was localized in the nucleus. Ectopic expression of TaARF4-A in Arabidopsis caused ABA insensitivity, shorter primary root length, and reduced plant height (PH). Through expression studies and ChIP assays, TaARF4-A was shown to regulate HB33 expression which negatively responded to ABA, and reduced root length and plant height by repressing expression of Gretchen Hagen 3 (GH3) genes that in turn up-regulated IAA content in Arabidopsis. Association analysis showed that TaARF4-B was strongly associated with PH and root depth at the tillering, jointing, and grain fill stages. Geographic distribution and allelic frequencies suggested that TaARF4-B haplotypes were selected in Chinese wheat breeding programs. An amino acid change (Thr to Ala) at position 158 might be the cause of phenotype variation in accessions possessing different haplotypes. Ectopic expression and association analysis showed that TaARF4 is involved in root length and plant height determination in wheat. This work might be helpful for selection of wheat genotypes with optimal root and plant architecture.