The thing is, although a 5 gallon sounds small, if you only had two ADFs in there the tank would look basically empty! They don't actually move around that much and are often hiding away somewhere, as you know...so it's not like they need several feet of swimming room like fish, as they prefer to crawl around the substrate anyway. From my experience of having them, I cannot see a problem with putting 4 ADFs in a 5 gallon tank. As you say, it's going to be roomier for them than where they are at present - and they have survived their present conditions ok so when they are moved they will have the place to themselves.
If there are not going to be any fish in the tank with them, then water stats aren't going to be a concern, especially if the tank is filtered and maintained regularly as you would your fish tanks.
Can't say I've ever noticed my ADFs being aggressive. Do you mean aggressive towards each other or to tankmates? Sometimes it might look like they are lunging and biting at a tankmate but they are more than likely searching for food as they don't see too well.
Now, as for removing them from the tank - have never heard that about the nets being dangerous (the LFS net the frogs anyway) - but I can imagine their little claws might get caught in the net. When I moved my frogs from one tank to another, I first tried a net but no sooner did I catch one when they just jumped out before I had a chance to whip the net out of the tank! So eventually I used a little plastic pot that had a lid. Took some time to get each frog into the cup and you have to be ready with the lid (inside the tank) to stop them leaping back out LOL!
No, not an easy task and takes some patience
Good luck and would love to see some pics of them in their new home!
Athena
ps - have a read of the pinned topic about ADFs by Azaezl - it's very informative and she states that she has kept 5 ADFs in a 5 gallon with no probs.