Responding depends on how you feel about it. If you are somewhat detached from this relationship, and that means you are not hopeful, or care to reunite with her, you can update her. But you are under no obligation to do so. Only you can decide, after knowing her for four years, whether you think she's genuinely interested in how you are doing, or if she wants to keep you on tenterhooks because of guilt, an undying need to appease her curiousity, or regret that she let you go.