Zone 4 is at the bottom of 'Northwind's' hardiness range, and 5' is as tall as it gets in my zone 7 garden. Unless you have all conditions perfect, I doubt it will reach 5' for you...but it's possible.

It doesn't flop for me, unless it's hit by wet heavy snow. It's extremely erect and upright in habit.

The ideal way to plant a hedge would be two rows, with the plants staggered and 3' apart. That means you'd need ~14 or 15 of them. Because the plant is so narrow at the bottom, you'll need the staggered rows in order to provide maximum coverage.

I'd suggest amending the soil at least somewhat, with organic matter like pine bark fines and some very coarse sand.

I have a screen of Northwind grass planted four years ago. It is now approximately 5'6' tall and up until two months ago stood tall and erect. However, following the intense heat and thunderstorms, it has flopped outwards from the center and does not seem inclined to stand back up this year. I routinely cut it back to the ground every February. If anyone knows a reason for the "flopping" I would love to hear it.

It's been a tough year for my grasses, between heat and drought followed by heavy storms/rain and high humidity. Several of my tall grasses are singed on the ends, and I've had to tie them up with green twine to keep them from flopping.

My guess is it's the weather. I keep asking for temps in the mid-70's and a quarter-inch of rain at night, every other night, but I'm not getting it.