Z4 is pretty cold for hydrangea macrophylla :) Our climates are so different, but I wonder if you wouldn't do better if rather than try to keep it growing, you just protected it from getting quite so cold.

Dan Hinkley of the former Heronswood had this to say about growing containerized hydrangeas in harsh winter climates - "move containers into an unheated porch, garage, or greenhouse during the winter season, protecting only during the coldest temperatures". Do you have someplace you could let it go dormant, then cover with blanket, table cloth, burlap during most severe freezes?

'Let's Dance Moonlight' makes for a very nice potted specimen ... for winter, I keep mine in an unheated attached garage that doesn't drop below 20 F.

Terrance