Yes, in that there are no intermediate states between those four bases, one can refer to DNA as a digital system by definition.
Google Wrote:dig·it·al/ˈdijitl/
Adjective: Relating to or using signals or information represented by discrete values (digits) of a physical quantity, such as voltage or magnetic polarization, to represent arithmetic numbers or approximations to numbers from a continuum of logical expressions and variables: "digital TV".