This is not normal behavior for a cygnet this young to be attacked by the male unless there is something wrong with the cygnet.

Placing the cygnet in another pond with other swans will surely get him killed or severely injured. The only alternative you have is to contact a wildlife rescue facility to see if they can intervene, capture the cygnet and raise it until it is approximately 9-10 months old and large enough to fend for itself. Not to intervene, may mean death by the male parent.

A 4 month old swan does not have the size, feathers or general strength to survive such inclement weather especially alone. So, the best advice we can offer is to capture and raise the young bird in a wildlife facility until it can be released in the spring. The Regal Swan