For words that only have 3 letters, there is no 4th letter. For example with the word “add”, there aren't any much more letters Later on and therefore should be the word "add". The word “addict” is within the BIP39 wordlist but you might have applied “addi” for “addict”. Mnemonic https://telegra.ph/Unlocking-the-World-of-BIP39-A-Comprehensive-Guide-to-Mnemonic-Expressions-and-Word-Listings-01-17