That size tank is not going to be big enough for four tree frogs. If you are restricted with such a small footprint maybe you could build something custom like John does with his 10 gallon conversions. You should try and keep it 10 gallons per frog if you can or as close as you can. Its just better for them to have as much space as possible especially with height. Do they even make a 40 gallon long? That way you could stand it on its end for the height. The standard 40 gal is probably bigger than 30 cm wide though. Just some ideas. Check out Johns article on his 10gallon conversions that he uses for his dart frogs just to give you some ideas. It might even cost less than the Exo-Terra.