For words that only have 3 letters, there is not any 4th letter. By way of example with the word “include”, there won't be any more letters afterwards and so must be the word "increase". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” https://thesocialcircles.com/story4488598/the-2-minute-rule-for-bip39-all-words