Don't you think 15 gallons is a little small for 3 to 4 FBTs? They wouldn't have much personal space, and they can be territorial buggers, especially when mating. I'd go with at least a 30 (29). If you're worried about the cost, you can usually find this size at garage sales or flea markets. You shouldn't be getting the frogs just because you have an extra tank and want to do something with it. The animal's needs should always come first.
I don't know if you've kept frogs before, so I"m going to act like you haven't, okay? I do apologize if you've heard this stuff before. Only use RO water. Frogs are very sensitive to the chemicals in tap water, and it can damage their skin very badly.
The substrate I've seen used most often is gravel, but you would want to pick a large enough size that they aren't tempted to eat it, or eat it accidentally when going for a cricket. River rocks are nice, as was suggested. You could also plant some easy plants in there, like a fern or two, a couple pothos or wandering jew cuttings, a very small bromeliad, or a polkadot plant. Just make sure the plants you pick are fairly sturdy. Don't worry if you know nothing about plants. Pretty much all plants love terrariums, and will do very well in them with little to no effort on your part. You could keep them planted in their pots if you want, and just bury the pots in the substrate.
Make sure the water part isn't deeper than 3-4 inches, as they aren't that great of swimmers. Water plants are great. Try anubias nana, java moss (will eventually spread onto the land part and make a nice, soft green carpet for your froggies), and perhaps a floater like water lettuce. The frogs will probably lay their eggs in the java moss, and the tadpoles will enjoy hiding in it.
You will probably need a heater for the water, but don't put it in the water part or the FBTs will try to sit on it and burn themselves. Stick it in the fluval, if you can. If you can't do that, try to cover it well with rocks or a piece of driftwood or something.
Keep in mind that FBTs should have a lifespan of 15-20 years, so this is a long term pet investment. If they are living less than that, you are doing something wrong.
I've linked another thread on another forum that has more info. for you below.
Fire-Belly Toads thread