If you are looking for the gallery, see M4A1 Gallery.
M4A1 assault rifles can be seen abandoned with several dead soldiers and other military personnel around the Centre for Disease Control Headquarters (CDC) in Season 1 Episode 5, Wildfire. It is seen in the opening sequence of Season 1 Episode 6, TS-19, when Shane Walsh (Jon Bernthal) witnesses soldiers executing infected nurses, doctors and paintents inside the hospital that Rick Grimes (Andrew Lincoln) was put in after he was shot. An M4A1 is also carried by Dr. Edwin Jenner (Noah Emmerich) later in the same episode. When Dr. Edwin Jenner first confronted Rick's group he allowed inside while he was holding the M4A1.