At what age do Swans leave their signets?

Some young signets have been left on a local pond, is this natural for the mother to leave them at approx 4 months old?
No, that sounds a bit young to me. The cygnets usually stay with the parents right through the winter, because at migration time they need to learn when to leave and where to go. They learn the route by following the parents.
They then return with their parents the next spring so that by that time they know the route. They tend to return to the nest area because its familiar, but the parents become less tolerant as they prepare for the next brood.
Then the cygnets disperse, mate, and find a nest area of their own.
It may be that the mother has met with an accident. You could contact the local RSPCA and see what they say.
you ever heard the likes of a pack o' whinin' signets? it's the leading cause of birdie abandonment.heh heh.
I think it is natural for swans to leave their cygnets when they mate and then start to prepare for the next years breed though i am not a proffesional in these matters so i cannot be sure