I love both of those miscanthus, but in zone 4b, I wouldn't recommend either one to you.

For a hardier, narrow leaved maiden grass, try Miscanthus Sarabande. It survives in zone 4 and has a similar look to Gracillimus. Blooms earlier, which is a plus for colder zones. You'll see plumes as early as August, where Gracillimus may not bloom in time before first frost.

Neglected to answer re the size of those miscanthus. Sarabande will be about 6' tall and you can usually figure on half the height in girth, so maybe a 3' wide clump. Morning Light, about the same size (a tad shorter), and Gracillimus a bit taller. They'll all be upright arching grasses, pretty good form and minimal flopping.

i live in z4------purchased and planted 'Sarabande' a couple of years ago. i checked the grass regularly to make sure it was doing fine. had no noticeable signs of problems. it didn't make it through its first winter. i was bummed (as it was rather "pricey"). i kinda shy away from miscanthus now---though, i see many of the garden centers in the area carry, at least, 'morning light'---it is tempting.

Sarabande is one of the hardier miscanthus...supposedly makes it in zone 4, but you never know.

You should try Purpurascens. That's a great cold climate miscanthus.

I'd be really surprised if Morning Light survived a cold zone 4 winter, but I've been surprised before.

I grow Miscanthus sinensis 'Morning Light' and it has been hardy for me, it has returned for 8-9 years in a row, and blooms every year in October with its reddish flower heads at about 5 feet tall. I love the fine varigation on the foliage, the vase-shaped form, and its winter presence. It is slower to get going in spring, being a warm season grass, but it has been totally hardy here in zone 4b, south central WI. Plant it in the spring for the best chance of establishing clumps before winter.