Four feet away, alongside or out front? My Autum Clematis is a real grower, sending long tendrils along the fence. I kind of lead, fold and wind the tendrils where I want them to grow along the fence. Can get rather thick at the fence top, though not usually by the roots as much.

4ft in front would be safe if you control the tendrils somewhat. 4ft beside the clematis would be a nothing distance to reach over, probably cover the Peony.

Other Clematis are less vigorous, could be trusted near Peonies. That Autumn Clematis is a real busy grower, gets long and thick. Even trimmed hard yearly in spring, mine grows incredibly every summer. They do seem to get thicker, more growth every year as roots develop. Of course the flowers and smell are REALLY nice in the fall!

How about the seeds? I heard the seedings from SA Clematis may be crazy sometimes. I am worried it may 'overpower' the peonies....