A fifth suspect, Liz Hernandez, had already pleaded guilty for her part in the murder.
However, officials tell NewsWest 9 that they filed a motion for acquittal on Thursday.
Last May, police said Lamb was beaten and forced into a car then shot in the head and back.
The 22-year-old victim's body was later found inside the SUV in an alley on Neta Place.
A motion hearing for the defendants has been scheduled for mid-May.
Meanwhile, federal authorities are still searching for two people they believe were involved in the drug operation and murder.