For words that only have three letters, there is no 4th letter. For example with the word “increase”, there are no extra letters Later on and therefore has to be the word "include". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for “addict”. You https://bookmarkspecial.com/story19637779/the-2-minute-rule-for-bip39-all-words