From Military.com:

“The government is seeking to procure M4A1-Plus (abbreviated as M4A1+) components as non-development items … for improvements to the M4A1 Carbine,” according to the March 13 document posted on FedBizOpps.gov. “It is anticipated that the M4A1+ components will be evaluated as a system. The system must then install on/interface with stock M4A1 Carbines.”