The Illinois State Historical Society has ruled repeatedly against recognizing 406 S. 4th as Wyatt Earp's birthplace. The committee established in 1956 to determine the birthplace listened to the arguments, then chose to place the marker at the city park. The Illinois State Historical Society has confirmed that decision repeatedly.
Article from Galesburg Register-Mail, March 17, 1994: "...no concrete evidence that Earp was born at a home at 406 S. Third Street."
Article from Monmouth Review-Atlas, May 30, 1996: "It appears that somebody has been mislead."
Article from Monmouth Review-Atlas, May 31, 1996. Executive director of the, Jon Austin, said: "If you cannot show additional documentation, we have no choice but to stand by our original decision."
Letter to Editor of the Review-Atlas, June 6, 1996
There was, however, one Earp expert who declared without qualification that the birthplace was at 406 S. 3rd: Glenn Boyer
For those who are unfamiliar with recent scholarship regarding the Earps, Boyer is widely regarded as unreliable. Google the controversy up.