Warren Buffett is considering exiting his four decade investment in Graham Holdings Co (NYSE:GHC) (formerly the Washington Post Co.) according to a regulatory filing. According to the SEC Filing, Berkshire Hathaway Inc. (NYSE:BRK.A) (NYSE:BRK.B) would swap its entire stake in Graham for some combination of assets or companies that Graham owns and Berkshire Hathaway common stock structured as a tax-free split-off. The filing specifies that there is no specific deal on the table right now, and that the Graham Holdings Co (NYSE:GHC)’s board of directors hasn’t been consulted about a potential deal even though it would require their approval, so this…