A for-profit hospital group from Tennessee has expressed an interest in buying Landmark Medical Center. The Woonsocket hospital on Wednesday signed a nonbinding letter of intent with RegionalCare Hospital Partners, of Brentwood, TN, owner of four community hospitals in Alabama, Iowa and Ohio. But this latest twist in Landmark's years-long search for a buyer is not the end of the road. Jonathan N. Savage, the court-appointed special master overseeing the financially failing hospital, is urging other prospective buyers to try to top RegionalCare's bid. RegionalCare was formed only 18 months ago, but says its executives have extensive experience running hospitals, many similar in size to Landmark. RegionalCare's primary shareholder is Warburg Pincus, a large private equity firm.