Well let me put my two cents in:
Goldfish: cold water, dirty, can handle bad water quality better, likes living in groups
Bettas: warm water, clean, cant handle bad water, to smart, prefers living alone.
So as you can see bad combonation. I would buy him a tank with heating and filtration. The owners shouldn't have put him in a small jar.
Also: 4 goldfish will need at least 60 gallons of water (20 gallons for the first one and 10 gallons for each additional one).