PIF4 (PHYTOCHROME INTERACTING FACTOR 4) is a transcription factor involved in the phytochrome B signaling pathway. Interacts with APRR1/TOC1 and PIF3 and binds to EGL2 and RGA. Alternative names: AtPIF4, Basic helix-loop-helix protein 9, AtbHLH9, bHLH 9, SRL2, Short under red-light 2, EN102, Transcription factor EN 102, bHLH transcription factor bHLH009.