So...you need a grass which will hit at least 4' tall, and be much wider at the top than the bottom, and live in constantly wet soil. Is that right? How's the sun exposure?

Typha (Cattail) has a couple of species which thrive in wet sunny spots, and are hardy in your zone, but they are pretty well upright, and not fountain shaped.

There are some Carex species, like C. muskingumensis and C. pendula which could work.

Chasmanthium latifolium, AKA Wild Oats could work.

Calamagrostis x acutiflora 'Karl Foerster' could work, but it's also very upright, and not fountain-like.

You could also consider some hardy Bamboo selections.

The common fountain-shaped grasses, like Miscanthus and real Fountain Grass (Pennisetum) will only take wet soil if it drains well and quickly. Same with Switch Grass (Panicum virgatum).