'Unprepared' Kim Kardashian and Kanye West 'Are Both Very Excited' About Baby No. 4: Source

Kanye West and Kim Kardashian West can’t wait to be a family of six.

The superstar couple are expecting their fourth child, a son, via surrogate, “in a few weeks,” according to a source who adds to PEOPLE that although the mom-to-be “feels unprepared” she realizes that “as soon as the baby is born, she will feel better.”

“She thinks it’s difficult to prepare mentally for a new baby,” the insider says, explaining of the reality star’s 14-month-old youngest child, “She felt the same about Chicago but the second Chicago arrived, everything worked out great. Chicago fit in right away. It was like she had been a part of the family forever.”

“Kim and Kanye are both very excited,” the source continues. “Chicago grew so quickly, so they are happy about having one last baby in the house. But this is it, they both say. Four kids is the perfect number for them.”

Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Parents newsletter.

RELATED: Kim Kardashian Shares Sweet Saturday Morning Family Selfie — With All 3 Kids!

Kim, 38, told the New York Times in an article published Saturday that she is “frantically” preparing a nursery for her and husband West’s baby boy inside the couple’s $60 million mansion.

“I’m frantically trying to get the room ready,” said the Keeping Up with the Kardashians star. “It’s madness, but the best madness.”

The newspaper, which noted that the baby is due in May, added that Kim and West, 41, keep a “mood board” in their living room featuring “a collection of looks they like, or tweaks they want to make to Kim’s cosmetic, fragrance and accessory lines.”

PEOPLE confirmed in early January that Kim and the rapper are expecting their fourth child, a boy. The baby on the way will join siblings Chicago, Saint, 3, and North, 5?.

RELATED VIDEO: Kim Kardashian Reveals How She’s Feeling Ahead of Baby No. 4’s Arrival

Sunday’s season 16 premiere of KUWTK opened with a rare appearance from West in his first on-camera interview for the show. Then, he and Kim revealed their big news over a meal with Kim’s mom Kris Jenner, her boyfriend Corey Gamble, sisters Khloé and Kourtney Kardashian, Kourtney’s ex Scott Disick and their two older children: Penelope Scotland, 6?, and Mason Dash, 9.

“We have an announcement to make,” Kim declared to the table. “We’re having a baby. We’re having a boy.”

The KKW Beauty founder added that, like her younger sister Kylie Jenner, she wanted to keep the birth of her and West’s fourth child (and second via surrogate) a secret.

“We don’t want anyone to know until it happens,” she said. (But it turns out her oldest child North was already going around telling people at school that she had a baby brother on the way.)

“I’m low-key freaking out,” Kim admitted later to Kylie, 21. “I didn’t think it was going to happen. I thought, like, ‘Oh, it probably won’t even take.’ “