The combination of the digits pronounced in Fujianese sounds similar to an insulting remark meaning "Lousy, goddam Mainlanders".  It has become an internet usage for those China-haters to address mainlanders, but it is quite inappropriate and unacceptable to abuse someone from Chinas Mainland to his or her face. 

Reports said a guy called two women names with "426" at a Taipei subway cabin in the evening of New Years Eve simply because the two women were talking with each other in Mandarin of Mainland accent.  Some people may criticize the guy with moral standard, but Id rather think from different angle.

The act of that rude guy represented one of the lowest, dirtiest natures of human beings: bullying over the weaks.  I may be a little bit paranoid, but such an act is a "heritage" from Japan.  See what Jap troops had done to bare-handed POWs and civilians in China and Sontheastern Asia during WWII, you will realize those China-haters in the island, including Cai and DPPs, are really descendants of "royal Jap citizens".  Intrinsically, they are nothing but a band of cowards.