It is suppose to be okay in zone 4, you should be able to plant it outside and it will return. I hate that plant--in zone 7 it is too invasive and every little bit of root turns into a plant--but mine was in the ground. I hope this last round of round up has killed the ones that have come up 10 feet from the original. I would plant in a pot just to be safe. A floating planter would be the best way to go because it is pretty.

I doubt they would survive if you just leave them in the planter unprotected. I would plant the whole floating planter in the ground and mulch around it for the winter and then dig it up again in time for the plant to start to regrow in the spring. You're trying to replicate what the plant would experience in the ground. But this is a highly invasive plant, so I would make sure to get it out of the ground in the spring before it has a chance to make a home in my garden!