The actress and her husband Brian Keathley welcomed son Sky LynLee on 11 December (13).
Kramer posted a photo of her newborn alongside daughters Gavin and River and 19-month-old son Hart on Wednesday (11Dec13), 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 (13).