Turkish tech site Donanim Haber claims ATI will introduce the Radeon HD 4890 graphics card on April 6th, 2009:
AMD-ATi will launch Radeon HD 4890 in April 6th. AMD-ATi Radeon HD 4890 is just a 'super clocked' 4870, still 55nm manufactiring process and same architecture just faster clocks. Clock speeds are yet to be decided but problably comes with 850 gpu and 3600MHz (maybe 3900MHz?) memory frequencies. Currently launch date of HD 4890 is April 6th.