The Official AGT Audition website says this:
Auditions for Season 4 of America's Got Talent are now over. All the acts have been notified.The show will premiere on JUNE 23 9/8C ONÂ NBC.Online pre-registration for season 5 (which will air in 2010) will start June 15, 2009. Check back for details.
Keep watching the website to audition for next season, if you were unable to get into this season's show!