How hopeful can a patient be with stage iv non hodgkin lymphoma on chemotherapy?

Depends on type: There are several types of non hodgkin's lymphoma. Most are very responsive to treatment. Some are diseases with a long history of intermittant treatments and relapses. Some are treated aggressivesly for cure. The details of the kind of lymphoma determine the likelihood fo success. There are some tumor gene profiles for large b cell lymphoma that predict outcome, but most are uesd for research.

Depends on subtype: There are many different subtypes of non hodgkin lymphoma. Some are highly curable despite their advanced stage (stage iv) , such as diffuse large b cell lymphoma. Others are very slowly growing and patients can survive for yrs or even decades, such as follicular or small lymphocytic.

It can be cured: And you are likely on retuximab too. Please ask your doctor why thaey have not made clear what your chances are, and why you should be optimistic.