Ayda revealed the birth of Beau Benedict Enthoven Williams to fans on Instagram.
She wrote: “On this Valentine’s Day, we would like to celebrate love in the most awesome way.
“As with Coco, he is biologically ours, but born via our same incredible surrogate.
“We are so blessed to have our healthy son safely in our arms and are officially complete as a family.”
Alongside the message she shared a photograph of the baby’s feet next to those of his siblings.
The couple’s daughter Colette (Coco) Josephine Williams was also born using a surrogate in 2018.
They also have a daughter, Theodora, born in 2012, and son Charlton, born in 2010.
Following the birth of Coco, Ayda explained that she and Robbie endured a “long and difficult path” trying for another baby.
The 40-year-old said: “Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful.”
Last year, Ayda admitted she was “desperate” to have a fourth child during an appearance on Loose Women.