Dear Felix
Reluctantly, I am writing regarding an unfortunate situation--we have not received payment on your account for four months. You are a reasonable person. Surely, you can understand that we can no longer carry this debt. Please contact me as soon as possible so that we may remedy this situation. Perhaps we may discuss an alternative payment plan.
I am confident that we will resolve this problem promptly and on reciprocally agreeable terms. I am looking forward to speaking with you.
Regards
Lydia