Zone 8PNW, Aconitum (Monkshood), Acanthus (Bears Breeches), Japanese Anemones, Delphiniums to name a few.

Annette

Persicaria polymorpha is a beauty right now, at least 4-5 feet in height, and beginning to bloom.

Geez, I wish my Aconitum were that big by now...

-Karen, I am guessing you want something that is over four feet tall by now? How high would you like it?

Knautia is a good choice. It lookes like a really tall Scabiosa. They come in maroon, pink, and lavender-pink colors. They are quite tall and airy, so you can see thru them. Upright habit with matte green leaves.
I have quite a few in various parts of my yard. The one in full sun is over four feet tall, and the one in part sun is pushing five feet.

Epilobium (angustifolium I think) is five feet tall right now, and will get to about nine feet tall by the time it is done blooming. It has Gaura-like pink flowers on long stems. Very nice plant, but be careful, it does spread a good deal. It sends out long runners. I like the plant well enough that I do not mind yanking a whole bunch in early spring.

I do not grow them, but if memory serves me correctly I think Fennel and Foxtail Lilies are up to about five feet or so by now (in zone 5).
CMK

I'm in your zone and these are four feet now:

Malva--various cultivars of M. sylvatica--M. sylvatica 'mauritiana' making an especially spectacular display

foxgloves

Baptisia

Amsonia is about 3' but will soon be 4'

Filipendula--4' but not yet in flower

One foxglove, because they didn't germinate better and where I planted them, one from last year was but not this year, Canterbury bells a little shorter, coreopsis, iris, maybe phlox, all lilies except daylilies. Am starting some delphinium from seed so don't know about that one, and none of my others are that tall, but I've got lots of different seeds sprouting, and most are supposed to be tall but I don't know how tall this time of year.

My Lilium longiflorum and Lilium regal are 5 ft. tall now.
A friend has Ostrich Fern that is 5 ft tall. She promised to bring me some this weekend.

aconitum carmicaeli is 4 feet now, Joe pye weed too, lots of clematis by the back will solve issues of height nicely or shrub roses, persicaria polymorpha is 4-5 feet tall

Persicaria 'Crimson Beauty' is about 11 feet now, as near as I can figure'. I'm zone 6 actually.

My Baptisia australis is at least 4 ft now and has been in bloom for about 3 weeks. Nepeta subssillis is also in bloom now and over 4 ft. Also my Monarda 'Raspberry Wine' is that tall, but not blooming yet.

Aruncus dioicus is also over 4 ft now in my zone 5 garden. I have some growing near a Thalictrum myself. Like the Baptisia, you'll need some room for this one.

Ive got hollyhocks that tall, not quite blooming yet. I call myself bordline zone 5/6, i moved about 30 miles north since registering for this forum, it makes a surprising difference.

Just thought of something else. Some of the Alliums are quite tall. I have Allium 'Gladiator" that tops out about 5 ft. Putting it in the back of the border would be a good thing as, for me at least, the leaves emerge so early that they gets badly frost bitten before blooming. Plants in front would hide that ugliness when those beautiful round balls appear. Left standing, they add a nice accent throughout the summer.

I'm in zone 5 Chicago. My phlox David is probably about four feet tall but not blooming yet. My peonies are that tall and blooming. Also, several of my clematis plants are that tall. Maybe a clematis on a trellis or other structure would work? Are you interested in a compact flowering shrub?

I'm in zone 5 (which I treat as 4b -- it's safer) in Iowa. Baptisia is 4+ feet and blooming. Veronica Goodness Grows and Filipendula Rubra (Queen of the Prairie) are both over 4 feet. My Joe Pye Weed is a little shorter at 3 feet, but I just put it in last year. I would think an established plant would also be at 4. For shade, Cimicifuga racemosa 'brunette' (too old to learn new genuses) is getting close to 4 as is Ligularia.

linnet, just FYI, I doubt the plant you refer to as Veronica 'Goodness Grows' is 'true-to-name' if it's over 4 feet, as this plant should only grow to 12-14' in height when in flower.

Here is a link that might be useful: Missouri Botanical Garden

Zone 7.

Agastache 'Golden Jubilee' is four feet tall. As is Agastache 'Salmon & Pink'. Both are blooming.

Joe Pye Weed is over four feet tall, but the flower buds haven't opened. Some of my echinacea are approaching that height. Magnus isn't blooming yet, but most of the "Big Sky" series are.

My tallest Bee Balm is approaching 4 feet, but not blooming just yet.

à¹à¸±Hydrangea arborescens 'Annabelle' is a soft wood shrub that acts as a perennial. Mine is 4 ft tall and setting buds. I cut mine down to the ground every spring.

What a bunch of great suggestions. I've been googling like crazy.

Thanks to everyone who contributed.
Big group hug!
Karen

I'm in zone 5, 2 hours east of Chicago.

My Persian Cornflower is 4 ft tall & just came into bloom this week. The flowers are not huge, but they are interesting. I have Walkers Low catmint that has been in bloom for a few weeks that's close to 4 ft. & it's went crazy in a bush form. Not sure if it spreads like other mints or not. My Black Knight Butterfly bush is 4 ft. so far, but not even close to blooming now. I have one daylily plant that the leaves are 3 ft. tall & the buds are starting to shoot & they will get 4 ft tall. I don't know the name, but it's a peachy gold color daylily. Moonflower can get pretty tall if you have sturdy enough support for it & the huge, white blooms smell wonderful. It's not 4 ft tall now, by any means though.