Why not a rose vine on either side that meets in the middle? You'd need one that is hardy to your zone 4 climate so that you don't have to worry about preparing it for winter. It may require some trimming back of some dead canes in the spring but once established, it should bloom nicely for quite awhile in the summer if you get a rebloomer. Check with your local nursery for possibilities.

Another option would be the Clematis which comes in various bloom colors.  Plant a different variety on either side of the arbor to ensure a longer bloom season.  You'll need to ensure that it's feet are well shaded but the rest of the plant loves the sunlight. Maybe you could plant some kind of ground cover around the base of the arbor to shade and cool the clematis' roots.

Several other possibilities would need some very sturdy and much larger structures to support them so I limited the list to these two which I think would be manageable for the size of your arbor.

I hope this helps.