The series of photos included Banks showing off her art skills, hanging out with her younger sister and older brother, and some cute throwback pictures of when Hilary was pregnant.
Many fans and celebs flocked to the comments to share messages to the birthday girl, including Mandy Moore, Ashley Tisdale and Ireland Baldwin.
Matthew Koma, Hilary’s husband and father to Banks also posted a sweet video in honour of her birthday, revealing that he isn’t the only musical one in the family!
The lead singer of Winnetka Bowling League shared a video of his daughter singing along at a recent Harry Styles concert.
“Harry 4th Birthday Banks Bair. May you always sing with the most engaged and concerned look on your face. Love you more than you’ll grow up to love Dawes,” he captioned the video.
The couple both shared clips of Banks enjoying the Harry Style’s concert for her birthday over the past weekend.
“I’m so excited to go to Harry Styles right now," Banks declared in a clip on Duff's Instagram Story, before excitedly asking "are we going to Harry Styles?"
Hilary and Matthew also share a one-year-old daughter, Mae James Bair. Hilary also has a ten-year-old son, Luca with ex-husband Mike Comrie.
Hilary recently opened up about experiencing working mum guilt in August, when she was stuck working on set as her youngest child was home with hand, foot and mouth disease.
"None of my other kids have ever had hand, foot, mouth, so I've never seen it before and that looks awful," she shared, adding, "and I haven't been able to be with her all day because I'm at work."
"I love my job so much but this is just a little shoutout to working parents who have to leave their kids in times that don't feel natural, and it kind of goes against everything in your body to not be with them in times like that."