My elderly hamsters in the past have always liked pureed foods, making your own baby food but including protein.
I included boiled potatoes, carrots, broccoli, any veg really that can be softened and safe for them.
Some of their normal hamster food, even mealworms, for fibre and protein, added to some hot water. Add this to the boiled veg and blend well. I also added boiled chicken as my hamsters always loved chicken.
These can be placed in ice cube trays and freeze, I find them a good portion size.
Heat them thoroughly, then cool till their a good temp to touch, mine always prefered the food warm.
I wouldn't re-heat the food as a concern about bacteria growth.
I'm not sure about the condition, unless it's just old age.
Sounds like you really love your hamster and have kept him happy and healthy for many years. 4 is quite old for a hamster, so you must have done a really good job
