Four is not old for a generally healthy betta. Have you tried the pea thing (half a thawed mushed pea, no other food for the week)? That helped a lot of my betta rescues with swim bladder and bloat issues (the swollen belly could be bloat, which will affect the swim bladder).
As far as euthanizing fish, I always did the freezing thing. I usually only did that if it was something untreatable and the fish was visibly suffering (such as dropsy). I've had fish with impaired swim bladder function that still lived well for over a year, especially one old betta of mine -I gave him a smallish (1gal, one of those self-contained units with the lighting and filter built into the lid) warm tank with a lowered water level and lots of plants, feisty little guy had a great appetite despite swim bladder issues and a crooked spine.
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