For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “increase”, there aren't any extra letters afterwards and so should be the word "increase". The word “addict” is while in the BIP39 wordlist however , you would have applied “addi” for “addict”. https://getidealist.com/story21402899/the-invalid-bip39-phrase-diaries