Fourth Edition will be a new edition of D&D, in the same fashion as the Third and Second Edition. They will be releasing the core books (the PH, DMG and MM) this June. There will be a number of changes to the rules, classes, races and background in this upcoming edition. Much of the information on the specifics of the upcoming edition remain unrevealed, but you can find much of what is known on the 4th Edition info page on this site.
As others have noted, Wizards of the Coast will be offering a subscription online service that is planned to allow tabletop gaming over the net. From what I've heard, it will likely act similarly to Fantasy Grounds and similar programs. However, it is not planned to somehow replace or eliminate standard face-to-face tabletop gaming.