Posing with the happy little boy – who wore a cool dinosaur tee for his big day – both Duff, 28, and the NHL player, 35, looked happy.
“We are so proud of this boy,” Duff wrote.
The star also shared a photo of Luca alone, writing alongside the image, “You….are magic my little big boy.”
She added, “Happy 4th birthday. My heart just explodes for you.”
At the time of their split, Duff’s rep told PEOPLE that the pair remained “best friends” – something that seemed mostly true throughout the first year of their breakup, as they even vacationed together as a family.
“We are a unit and we’re going to be a unit for the rest of our lives,” Duff told PEOPLE of co-parenting in April 2015.
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Even with Comrie still in the picture, Duff has said that being a single mom is hard work. “When I’m really busy with work, I feel extremely guilty,” she told Redbook in late 2015.
She added of balancing her mommy duties and Hollywood career, “It’s hard to say that you really can have it all. I think you can to a certain degree, but fully loaded, pedal-all-the-way-down is tough. You’re missing out on something, either on the career or on the family.”
— Lindsay Kimble