There's no reason to believe that there's anything other than a four-year extension here, but are you certain about the CBA not allowing an option for the fifth year? Interestingly, the CBA FAQ seems to be mute on the subject.
A maximum qualifying offer to an RFA can't have any options, but this is not an MQO. Extensions to veteran contracts are explicitly allowed to have an option, but there's no comment either way on extensions to rookie contracts. Now, in order to give a fifth-year team option you'd have to make Cousins the Designated Player, but I don't see anything disallowing an option on the final year of a Designated Player extension. Might be an interesting question for Coon.