Hi bluebetta, and welcome!
Four years old...wow! That's pretty good for a betta; excellent, actually! I've read that bettas kept in vases or bowls may live longer than those in tanks, because of the cooler water keeping their metabolism lower. However, I would only keep, or recommend that others keep, bettas in the tropical conditions that they are most comfortable in.
I looked at the pictures and it may be a tumor of some kind. I don't know what kind of medicine to recommend, unless you can see something like small white dots, white cottony patches, or ragged fins. I would not use BettaFix again. Some people report that bettas do not do well with the active ingredient in BettaFix, or MelaFix. While this ingredient is fine for other fish, it may harm the labyrinth organ in fish like bettas. I used MelaFix on my betta once, and it didn't help him. He got very listless. So while some people may use BettaFix, and like it, I didn't find it helpful, and since you didn't either, I wouldn't try it again.
Instead of medicine, I would change the water in the bowl 50% every other day. You don't want to stress him out with too many water changes. But first, I would get all the BettaFix out of the water, and then do the 50% changes. This will hopefully keep him comfortable. Can he come to the surface at all? Bettas do need to supplement their gills with the labyrinth organ, so they do have to get to the surface. If he can get to the surface comfortably, then he may perk up.
Unfortunately, my first betta lied at the bottom for almost a week, and couldn't come up for air or food, and died shortly afterwards. It may just be your betta's time
I don't know of any remedy once they get that way. Try to keep him comfortable, and in an area where the temperature stays stable. Try the water changes to get the BettaFix out of the water, and keep your fingers crossed!