Instead of adding a fifth defensive back against all three-receiver packages, the Patriots sometimes subbed out a safety (Duron Harmon/Steve Gregory) in favor of a third cornerback (Kyle Arrington). So while they technically remained in a base defense with four players in the secondary, it had a sub-package element to it.
https://www.patspropaganda.com/instead-of-adding-a-fifth-defensive-back-against/