Just like people from any other countries, people in China traditionally associate luck with various numbers. In Chinese culture, certain numbers are believed by some to be auspicious (吉利) or inauspicious (不利) based on the Chinese word that the number name sounds similar to.

Because of the supposed auspiciousness of certain numbers, some people will often choose, attempt to obtain, or pay large sums for numbers that are considered to be lucky for their phone numbers, street addresses, residence floor (in a multi-story building), driver's license number, vehicle license plate number, bank account number, etc.

Lucky numbers are based on Chinese words that sound similar to other Chinese words. The numbers 6, 8, and 9 are believed to have auspicious meanings because their names sound similar to words that have positive meanings.

6 - "Liu" Do everything smoothly.
8 - 'Ba' has a similar sound to 'Fa' （发), which means to make a fortune. All business men try to get everything with "8". Like phone #, license tags, etc.
9 -"Jiu" sounds alike to the Chinese character (起) (Pinyin: qi3) meaning arise.

Unlucky number:
4 - "Si" sounds like 'Si' (death). So when people choose car license tags, phone numbers, and room numbers, they would avoid it.