Pam, Eucalyptus can be coppiced to get a bushy form. But if what you want is a tree-shaped tree of reasonable size, then what you need to do is get a less vigorous species as, at 4m in 3 years, it sounds like you have one of the vigorous ones. You could get rid of it now while it is practical to do so, and plant a less vigorous one so you can keep a tree-shaped Euc. The ones commonly sold are mostly the vigorous ones, you have to go to a specialist nursery, or buy seed, to get a less vigorous one, but there is something of a choice when you do go for the unusual ones. Quite why the British nursery trade hasn't yet gone in for the garden-friendly Eucs that do exist is one of those mysteries. Unfortunately both the really specialist Euc nurseries in Britain closed recently. E gregsoniana is the prime example of a lovely, less vigorous one, but now very hard to find. But I have 3 unusual Eucs in my small garden; one of them (E mannifera praecox) is growing at the same rate as a small birch (3m after 8 years), and the other two (E nova-anglica and E pulverulenta) are behaving like small shrubs.

http://www.angelfire.com/bc/eucalyptus/ is a very useful site on growing Eucalyptus in cool climates (Author is from US Pacific NW). It includes instructions on how to coppice, and a catalogue of the (remarkably many) hardy species.

Do not prune it now unless you want to risk losing it. Frost getting into the cut wound can kill it. The best time to prune is spring after the risk of serious frost.