4 in Chinese sounds like "death" while 8 is the lucky number relating to wealth and money. I am told that 16 means something similar to "easy road to...'. Therefore easy money is preferable to easy death! ;-) Chinese speakers may be able to explain it better than me.
That said not all 164's beecame 168, there are still a few 164 badged cars knocking around here, so not everyone was superstitious!