"production ready app in iOS, Android, website, and server setup as well as website/app design and logo creation and branding" in 4 months at 18 hours a week is an unreasonable scope of work, at least for me. I would not be employed under these terms unless my hourly rate was in the thousands or tens of thousands, and even then I would be concerned about the impact of four high-stress months on my health.
You are going to encounter these kinds of unreasonable requests throughout your career. The ability to explain in layman's terms and plain English why this isn't possible and to present an alternative is invaluable and it's part of what you're being paid to do. It is always better to risk disappointing the client in the beginning, by having this conversation, then it is to disappoint them in the end, by failing to deliver.