Atypical and probable non DNA-binding bHLH transcription factor that regulates light-mediated responses in day light conditions by binding and inhibiting the activity of the bHLH transcription factor HFR1, a critical regulator of light signaling and shade avoidance. Forms non-functional heterodimers with HFR1, causing liberation and activation of PIF4 from the transcriptionally inactive HFR1-PIF4 complex. Interacts with HFR1. Circadian regulation with a peak of expression at midday. Gain-of-function mutants kdr-D (T-DNA tagging) show long hypocotyls, pale green and slightly narrow leaves, elongated petioles and early flowering. They are not sensitive to the gibberellin inhibitor paclobutrazol during seed germination (PubMed:16786307). Belongs to the bHLH protein family.