How can I write my cat's name, TK-421, in Chinese? TK-421 (pronounced Teekay four two one) is the serial number of a character in a science fiction movie (and also my charming cat). What would be the most natural way to write this down?

TK-四二一?

TK-四百二十一?

Should anything be done with the letters?
TK-421, as a serial number, is ok in Chinese. But pronounce differently from English.