Re: Adding 4-Way Switch
Based on your description of the circuit, run a 4-wire cable (12/4 w/ground if 20A circuit, or 14/4 w/ground if 15A circuit) from the light fixture to the new location and put your 4 way switch at the new location (switch #3).
In the light box there should be two "traveler" wires from switch #1 that run directly to switch #2, not connected to anything else in the light box. Take the two wires from switch #1 and connect them to two of the wires to switch #3. Connected the two other wires from switch #3 to the two traveler wires going to switch #2.
There is no need to change connections at either existing switch.
NOTE: 14/4 is also labeled as 14/2-2 (and 12/4 is 12/2-2). I recently saw that it is now available at my local Homely Despot. With the 14/2-2 there will be a plain white wire and a white wire with a stripe. Since neither of these white wires will be used as a neutral in this scenario, you must wrap them with either colored tape or colored marker to indicate that they are not neutral. This is a code requirement.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.