I have the same problem, can't get 4x on my Via Abit VP6 board. I don't edit the source code, but use the module parameter NVreg_EnableVia4x instead, which does the same thing (type modinfo NVdriver to get all module parameters).
This worked with older nVidia binary drivers, but not anymore with 3123. E.g. option NVreg_EnableAGPFW works ok still.