Vallisneria, in particular Val. gigantea. Given the right conditions, it can grow to 5-6 feet, but can easily be trimmed with a pair of sharp scissors (important, because if the tops of the leaves are damaged the whole leaf can die.)
Other varieties of Vallisneria can also grow to the height you're looking for, or close, as well.
I've got a few Val. gigantea and Italian Val in pots in an 18" tall tank, without CO[sub]2[/sub], and recently had to trim both quite a bit as they were taking over the surface.
