According to Chinese sayings, good things come in pairs. Therefore odd numbers are avoided for birthdays and weddings. To avoid bad things happening in pairs, activities like burials and giving gifts to the ill are not held on even numbered days.
Also, the number four (四, sì) sounds like the character for death (死, sǐ). That is why the number four is avoided particularly on phone numbers, license plates, and addresses. For addresses that do contain fours, the rent is usually less and apartments on the fourth floor are typically rented by foreigners.