I was working a side job today and re-wired quite a few 3 and 4-way circuits. I have a good understanding on how each work, but when I hooked up a 3-way dimmer on a 4-way circuit, the light would come on only if I had a certain switch on. Basically, I put it where I thought was at the switch leg end of the circuit. I'm only assuming this because the first 3-way is a foot away from the panel then the 4-way and finally the last 3-way. I've heard several people say to put it on the switch leg end (granted it's wired for a traditional 4-way circuit) or the switch that is not the line side. I also heard it does not matter... as long as it's obviously not in the middle. What the heck is the problem here???
Was it a standard dimmer switch, rather than a 3-way one?
When replacing an on/off switch with a dimmer switch, you must make sure the type of switch (standard, 3-way, 4-way) matches. They make all three types of on/off switches, but I've only ever seen standard and 3-way dimmers. I figure the rarity of use for 4-way dimmers, combined with the rarity with which they would be purchased, make them not worth manufacturing.
Anyway, in a 4-way circuit (or more-way), you can replace either 3-way on/off switch (which will either be the first or last switch) with a 3-way dimmer. But it must be a 3-way dimmer, not a basic dimmer.
I refer you to some excellent diagrams on wikipedia to see how multi-way circuits work. Note the difference between how a 3-way and 4-way switch work. If you can find a 4-way dimmer switch, you could replace any 4-way switch in a multi-way circuit with it. Otherwise, it's either got to be the first or last switch.