The former tennis player celebrated the major milestone by sharing adorable snapshots of her kids to Instagram on Dec. 16 and captioning them, "4!" The first post showed Lucy hanging out on her outdoor playset and appearing to do some arts and crafts while the second post showed Nicholas smiling at the camera in a Mickey Mouse shirt and relaxing in a Spider-Man bean bag chair.
In addition to Lucy and Nicholas, Iglesias, 46, and Kournikova, 40, are parents to 22-month-old daughter Mary, whom they also call Masha. In September, Kournikova posted an Instagram video of all three children riding around in a toy car as Iglesias' hit song "Chasing the Sun" played in the background.
While the singer and the athlete tend to keep details of their family life private, they have given fans glimpses into their world both on social media and in interviews.
"When they watch that video, I think it does throw them off a little bit," the Grammy winner said. "'What is mom doing there, you know, with dad, you know? When was this?' It's actually, it's incredible to watch the expression on their faces. It's pretty cool."