I had stage 4 aggressive non-hodgkin's lymphoma last year, 6 months later I am cancer free! What are the chances of it coming back?

Better than even. This depends on variables including your age (younger does better), but even ten years ago, about 2/3 of people with stage IV (and stage is the major prognosticator) who were alive at one year were alive at ten (at which point it's unlikely to recur). Your oncologist has better numbers, but for any individual, it's either 0% or 100%. Cherish each new day, and best wishes.
Good cancer free. Good you are cancer free because you have intensive chemo thanks to your great oncologists . Dlblc ( diffuse large b cell lymphomas ) are aggressive ones. , next 2 yrs is critical time. Long term prognosis depends on age , extent of spread, more than one site, health status , elevated ldh levels.