Gilead Sciences Inc., of Foster City, Calif., said it priced senior unsecured notes in an aggregate principal amount of $4 billion in a registered public offering consisting of three tranches: $500 million of 2.35 percent senior notes maturing in February 2020; $1.75 billion of 3.5 percent senior notes maturing in February 2025; and $1.75 billion of 4.5 percent senior notes maturing in February 2045.