Mark-Paul Gosselaar has welcomed his fourth child, his second with wife Catriona McGinn -- find out the sex of the baby! Credit: Tibrina Hobson/WireImage.com

Cue the Zack Attack theme! Mark-Paul Gosselaar has welcomed his fourth child, his second with wife Catriona McGinn.

"So in love with our new baby girl, Lachlyn!" the Saved by the Bell alum, 40, tweeted on Wednesday, Feb. 18. "Born last night 2/17 Mum and baby are healthy and happy. 2 boys, 2 girls, too perfect!"

He later posted the first photo of his little tyke on Instagram. "She's a 'mouth-breather' like her Daddy," he teased. "My sweet little angel."

Gosselaar confirmed to Us Weekly back in December that the couple were expecting another little one. "I'm very excited!" he told Us at Disney on Ice's Let's Celebrate event in L.A. on Dec. 11. "We have told our families," he said of the baby's gender. "But we haven't told them the name."

She's a 'mouth-breather' like her Daddy. My sweet little angel.

A photo posted by @mpgosselaar on Feb 18, 2015 at 2:14pm PST

The Franklin and Bash alum also shares son Michael, 10, and daughter Ava, 8, with ex-wife Lisa Ann Russell. He and McGinn — who married in 2012 — welcomed son Dekker Edward in October 2013.

The new addition comes just weeks after the actor reprised his role as Zack Morris during The Tonight Show's epic Saved by the Bell reunion with host Jimmy Fallon. His former costar Tiffani Thiessen, who's already mom to 4-year-old daughter Harper, is currently expecting a baby boy with her husband Brady Smith.