Sonic HQ has put up what they think the four new games will be; they are NiGHTS 2 (a gimme), the Neo Geo Pocket Sonic game, Sonic the Fighters 2, and an unknown title. There should be more info soon.
Original Article 8/20/1999: The Sonic Team Updated its website today with some Japanese text and four new logos. See the page here.
What could this mean? Well, Gaming Age is reporting that the four logos represent FOUR NEW GAMES, of which NiGHTS 2 and an arcade title may be two! And the Sonic Team has gone on record saying that as well!
These titles would be shown in the Japanese publications Famitsu Weekly and Dreamcast Magazine in all of their 4 September issues, one at a time. Following September, the games would be shown at the Tokyo Game Show this fall, and at JAMMA, a show featuring arcade games.
Remember, at this point it appears to be speculation. The Sonic Scene will stay on top of this breaking story, but for now, if anyone out there can read Japanese, please Mail me and tell us what was said on the Sonic Team logo page.