If the cane or rhizome is still alive, it will regrow (from the top and/or the underground rhizome. Of course, it depends on the species of bamboo you have and your minimal temps this past winter. To be honest, I am not aware of bamboos hardy to zone 4 without some major protection (most of the Phyllostachys are hardy down to zones 6-7). Working against it was the fact that this was its first Winter in the ground. Plants take a while to get really established and if your winter was unusually cold or the cold was really prolonged, this doesn't help. But to get back to your question, I have had bamboos loose all their leaves (as a result of transplanting) only to regrow completely from the older canes once they got acclimated. If they are goners, and you really want to grow bamboo outside, you might try to protect the canes next time. I've heard some people bend the canes over to the ground and mulch heavily with hay before the Winter. (PS.: I have the opposite problem--trying to prevent the grove from taking over the planet (no joke).

First the leaves die back, then the buds for next year's leaf production, then the cane itself. If you're not sure, don't cut anything back until late May or June when you know for sure whether or not it's dead. : ) Don't be scared by topkill, if you've got snow cover, I'll bet that it will come back for you.

Thanks for your advice, njoasis & cuttlefish. So far, we have had a mild winter for our area. It has only gotten down to -7 F. so far, so maybe my canes will be OK. In the Fall, I put a thick mulch of leaves around the base of the plants, covered the mulch with plastic sheeting (suggested by someone) and weighted the plastic down with some bricks. I did bend some of the canes over and covered them with snow after we had a big snow storm the day after Christmas, but the snow all melted, so they are exposed once more. I'm just hoping it doesn't get any colder before Spring.