By hearty do you mean vigorous? Are you looking for something that will spread quickly, and how tall does it have to be? You would have more options in your zone than I do in Zone 4, but reliable ornamental grasses here would be Calamagrostis 'Karl Foerster', and the hardier Miscanthus. The latter would be taller, some very tall. Neither spreads much here, the quickest ground covering grass I've grown would be the variegated "gardener's garters", Phalaris arundinacea 'Picta', but it can spread too much if not contained. I'm not sure if that would be categorized as tall. If you can give a bit of background as to what you're wanting to achieve with it we may be able to make some other suggestions.
