Hi, I can help you with your question today.
I think we need to begin some more aggressive diagnostic testing to see what is going on with him.
It certainly isn't normal for a 4 year old cat to be so lethargic with a decreased appetite.
I am worried about kidney problems, liver problems, pancreatitis, etc.
I highly recommend having him rechecked by your local vet. This time, request that they run some blood testing to check his major organ function. I would also include the fPL test to look specifically for pancreatitis.
Hopefully these will be enough to give us a diagnosis so that we may begin a treatment plan.
Please let me know if you have further questions. I am happy to continue until I provide the service that you seek.