a Betta. that's all I would keep in a 4 gallon tank.
maybe an african dwarf frog.
correction: it would need fixing before a Betta could go in, looks like it has a mirror-back, that would have to be hidden completely so not to stress a Betta out.
possibly. but i would get your 15 gal sorted out before you buy any more fish. this means geting a larger tank first. all the tetras need to be in groups of atleast 5. but 5 schools in 1 tank is too much. true flying foxes grow to 6 inches and need to be in groups. i couldnt find info on the peltobagrus ornatus except that its a catfish likes to be in groups. untill you can get an appropriate home for your current fish YOU DO NOT NEED TO GET ANYMORE FISH PERIOD.