The following text is something I borrowed from SimsVIP (thanks for the info):
In an official email sent out to select fansites worldwide, EA states they will be setting up a conference call with all invitees to "hear directly from one of the developers about the exciting piece of news!". EA has also stated that after this conference call, all invitees "will receive a brand new image to accompany this special occasion", which they will be getting "approximately three hours before the rest of the world."
In my opinion, this may be The Sims 4. A much bigger possibility is an announcement of a new Sims online game (judging by the leak of some photos and closing down of Sims Facebook games). Then again, this online game can still very much be The Sims 4. The only other idea that I can think of is EA showing off the new name for the Untitled The Sims 3 Time Travel Expansion Pack, though this wouldn't fall into the "great announcement" they're making.
UPDATE 1: The Sims 4 domain EA registered a while back (among others) is active, and it redirects to TheSims.com homepage.
On a side note, another fansite, SimS, is leaking some interesting photos about the event on Monday. Now, I don't believe these photos are genuine, but this does raise interesting questions. Cavia (wheek • speak) 09:34, May 4, 2013 (UTC)
UPDATE 2: The Sims Russia teases on Twitter with the following tweets: 'Are you ready to play online with your friends?' and 'Want something new? You'll life will change next week, Simmer. '.
Again, on a side note, SimsVIP (along with other fansites) was too invited to this EA event.