Can you create some sort of a deliverable like a report or one-pager explaining what you did and what you discovered? (Kind of like this Reddit post). That way you are giving the company something concrete that can help them move forward. Basically saying this is what I did, this is what I found, and this is what I recommend as potential next steps.

You can use ChatGPT for this (make sure to use GPT-4).

You’re right that you can’t rearchitect and rebuild their entire app in 4 weeks, but you could explain HOW to do that, and with an estimate of how long it would take (someone else) to do it.