For words that only have 3 letters, there is not any 4th letter. Such as with the word “increase”, there aren't any more letters Later on and thus need to be the word "include". The word “addict” is in the BIP39 wordlist however you would've applied “addi” for “addict”. In https://bookmarkity.com/story18882250/little-known-facts-about-bip39-seed-phrase-list