Is there such thing as a 4-way dimmer?
I've used Smart Home's Insteon switches to do this.  The switches are a bit pricey, but work well.  You don't need to rewire the entire house, just replace individual switches as you need to.  One of the additional benefits is that you don't need to constrain yourself to how the original switches were wired up.  If you want to control a particular light that wasn't originally wired up as a three-way, its easy to do.  One thing that I did was add control of the outside lights to a tabletop controller next to my bed.  That way I can turn on the spotlights without going downstairs.