Can I fit 4 frogs in a 30 gallon tank?

I havent bought any frogs yet but will be going to a reptile expo over the weekend and know i want to get some dendrobates but also want to make sure they are healthy and happy. I have already purchase the Exo-Terra PT2607 terrarium; 18x18x24, about 30 gallons. The question is what number of frogs can I comforatably support within this environment? I wanted to have two types of frogs in one tank for aesthetics, specifically azureus and auratus. I know morph mixing or interbreeding is frowned upon but my studies have lead me to understand azureus and auratus will not breed with each other. Also, I understand azureus live best in sexed pairs due to hostile mature females. They tend to ignore auratus because they are not sexual competors thus alleviating potential sources of stress found from inter-species co-habitation. The remaining question, if 2 azureus can live with 2 auratus, is 30 gallons enough for them to live together healthily?

If anything I have said runs contrary to your investigations please let me know. I am less than a novice and eagerly seek the knowledge of those more experienced and am truly appreciative. Here's a link to pic's of the terrarium i am buying. Thanks a bunch.

exo-terra pt2607 - Google Search