For words that have only three letters, there is no 4th letter. As an example with the word “increase”, there are no far more letters afterwards and so must be the word "increase". The word “addict” is in the BIP39 wordlist but you would have employed “addi” for “addict”. bip39-phrase.com https://bookmarkssocial.com/story18891805/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition