I have a 4-way switch I want to replace with a dimmer. I understand I can put the dimmer at one of the other 3-way switches, but they're more out of the way. Does such a device exist? If not, what are my other options?
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.
Have a look at some of the remote control systems for lights that can dim. You don't need to limit your options to wired switches.
You would normally need to put the dimmer at the start or end of the multi-way chain where there is one active for the rheostat. Intermediate switches have two actives so for a dimmer to work there, it would need to be dual-gang, which is extremely unusual for a light fitting.