When Lucas is in the graveyard at Keith's and Jimmy's graves, there are two different months printed on the stones.
After Lucas' heart attack Lucas visits Keith's grave and then visits Jimmy Edwards grave. Jimmy Edwards gravestone shows that they died on March 1, 2006. Not only does that mean that all of season 4 happened in 3 months but when they made the 4 year jump to season 5 it means the show would be in 2010.
When Nathan is delivering Haley's soup, the news is covering a story on Dan's arrest. The news headline reads "Mayor Scott being held on bail', but the newscaster says seconds later that Dan is being held without bail.
When Lucas is visiting Keith's grave, you see part of the date of death, which says 23, 2006. Several seconds later as Lucas looks down on Jimmy Edwards grave, his death date says March 1, 2006. Keith and Jimmy died on the same day, during the school shooting.
At the end when Lucas is at the cemetery visiting Keith's headstone it says he died January 23rd 2006. When Lucas then leaves to visits Jimmy Edwards headstone and his says he died March 1st 2006. Jimmy and Keith die the same day right next to each other.
On Keith's tombstone it shows his death date as the 23rd, but does not specify a month. On Jimmy's it shows the 1st of April as his death date, but they died on the same day so it should be the same.