I'd put the large 4 inch long toad in the 20 gallon. A little more room for him is better. The more room the better! Also, I'd be willing to bet that the large toad will eat all your babies if you put them together! I only put my younger toads in with the older ones when it looked like the biggest toad could not put the smallest one's head in its mouth (comfortably)OR when I couldn't imagine the smallest toad fitting inside the biggest toad's belly. I may just be being over cautious with this 'rule' I have, but you also have to take in consideration that sometimes toads cohabitating with each other might mistake each other's wiggling toes as food. Injury can result because of that. I have a toad in which that happened to years ago. She is fine, but she has a permanent funny leg now. It was feeding time and my biggest toad saw her toes wiggle and thinking it was food he snapped at her foot and got it into his mouth. He had her leg in his mouth up to her "knee". She struggled as did he (not wanting his food to get away of course) and all that twisting and fighting resulted in her joint shattering. For this reason alone, its probably best to keep toads of relatively the same size together.
Best of luck with your new toads! Toadlets are really cute, aren't they!? 