For words that have only 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any more letters afterwards and therefore must be the word "increase". The word “addict” is while in the BIP39 wordlist but you would have made use of “addi” for https://throbsocial.com/story20874249/a-review-of-bip39-phrase-list