Your question is mis-stated, it appears to me that you are asking whether or not a 4-Way switch can be added to a 3-way circuit.
4-Way switches are 2 pole switches.
Normal On/Off switches are single pole SPST
while 3-Way switches are siingle pole SPDT
Please post a larger image schematic. I can't read it.
Here is a very recent similat Thread:
http://www.diychatroom.com/f18/help-3way-4way-lights-45017/
4-Way switches are 2 pole switches.
Normal On/Off switches are single pole SPST
while 3-Way switches are siingle pole SPDT
Please post a larger image schematic. I can't read it.
Here is a very recent similat Thread:
http://www.diychatroom.com/f18/help-3way-4way-lights-45017/