CT broke the news on social media Wednesday about their new boy, Wren Alexander Stephens, by saying, "For as long as I can remember, I've always wanted four children" ... adding she'd always play with 4 dolls when she was a little girl.

That dream finally came true for Chrissy, saying she reached out to a surrogacy agency in 2021 -- the year after she and her husband John Legend sadly lost their son Jack due to complications.

As we reported, Chrissy and John also welcomed a new baby in January, with John dropping the news at a private performance -- Chrissy's post says their little girl, Esti, was done through IVF, just like their other kids, Luna and Miles ... getting pregnant during their search for a surrogate.

Chrissy says Alexandra, the "most incredible, loving, compassionate surrogate we could ever imagine," was a perfect surrogacy match for the fam.

She notes the first embryo didn't take for Alexandra, and she underwent surgeries to make sure the second transfer would work ... eventually learning the process was a success. She says baby Wren was born on June 19.