The actress and her husband Brian Keathley welcomed son Sky LynLee, reported Hollywood Life.
Kramer posted a photo of her newborn alongside daughters Gavin and River and 19-month-old son Hart, along with the caption, "Meet my new little man! He joined our gang at 8:13am this morning, right in time for the holidays."
Kramer and Keathley married in 2005, and founded entertainment website GeekNation in 2012.
Little Sky's middle name is a tribute to Keathley's sister, who passed away in September this year.