Garrison brought a 4th Amendment claim because they did not have a warrant to search his apartment, but rather they had a warrant to search McWebbs apartment, and Garrison wanted to use the 4th Amendment to suppress the drug evidence.
Garrison argued that he should be able to exclude the drugs found in his apartment because the police did not have a warrant to search his place. He claimed they had a warrant for McWebbs' apartment, but Garrison wanted to use the Fourth Amendment to prevent the evidence from being used against him.