Where did you collect these? Location can help us ID things more accurately.
I think #2 is a Hypericum
species, not a Lindernia
. They look similar, but Hypericum
tends to get that silvery-looking sheen when you put it in the water. I've seen it grow submersed in the wild, but never heard of it being sucessfully maintained in an aquarium.
#4 doesn't strike me as a true aquatic, but I'm not sure what it is, so I could be wrong.