Four people were rushed to Nassau University Medical Center March 20 after being found by police in their homes suffering from breathing difficulties. Initial tests to determine if the four were victims of monoxide poisoning were negative and the investigation is continuing.     That incident occurred shortly after 2 a.m., on Redwood La., Massapequa. Two of the victims were found by police on the kitchen floor of the home and were unable to move. The Massapequa Fire Department and Fire Marshall HAZMAT units tested for carbon monoxide levels and found readings of zero percent. In addition, blood-gas testing on all four of the victims showed nothing unusual in their systems.     In addition, three of the five police officers who responded to the home complained of headaches after the assignment.