The 2 celebrities showered their daughter with love on Instagram Thursday, as their youngster turned 4 years old.

The supermodel took to the picture-sharing site to share a glimpse into Khai's 4th birthday bash ... which mashed-up a theme of rainbows, "The Mandalorian," and Disney Channel's "Descendants."

Khai blew out her candles from a massive Baby Yoda cake ... which had rainbow layers on the inside. There was also a big, inflatable slide, as well as an arts and crafts station ... proving Gigi spared no expense for her little one's birthday.

However, eagle-eyed fans noticed one party detail that seemed to reveal Khai's full name ... a fake royal scroll announcing the 4-year-old's enrollment at Auradon Prep, the school at the center of the 'Descendants' movies.

If you take a closer look, the name reads Khai Malik ... confirming for the first time that Khai has her father's surname.

While it's unclear if Zayn was in attendance for Khai's big birthday party, the singer took to social media to share his own tribute for the kiddo ... where he called her the "most important person in [his] life."

He continued ... "I love you more than words allow me to express, beyond proud to call you my daughter.. grateful for every second I get to spend next to you, as you become the incredible person I know you already are. Four years ago today my life changed forever and I wouldn’t be the man I am today without you."

Gigi and Zayn had an on-and-off relationship for 6 years, starting back in 2015. The twosome welcomed Khai in September 2020 ... only to break up for good the next year.

The exes appear to have a good co-parenting relationship ... even after Gigi moved on with actor Bradley Cooper.

Last year, Zayn shared on the "Call Her Daddy" podcast that "co-parenting is good" with Gigi ... noting they're focusing on having an amicable relationship for Khai's sake.

Gigi hinted at their friendly relationship on Thursday, seemingly referencing Zayn's song "Dusk Till Dawn" in the caption of her birthday tribute.