A 3-4g is fine for a Chinese firebelly newt (cynops orientalis). They are commonly available. Make sure it's not a japanese firebelly (C. phyrrogaster), as this would get too big. They are mainly aquatic, but do need a small land area - I have 2 in an 8gal with some white clouds, and they spend most of the time in the water but will come out to bask.
You could go for a firebelly toad... but ideally I'd get a 10gal as they like company (a standard 10gal is good for 5 or 6, and you can scale this down depending how many you want. I have 4 in an 8gal, but I'd still only go for one in a 3-4gal as such small tanks tend to be disproportionately high) remember, with frogs and newts surface area is much more important than volume.