Where is telephone country code 304?

Telephone country code +30 (dialed as 00 30 from many places) is Greece, but there are no valid numbers beginning with +30 4.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)