For words that only have 3 letters, there isn't any 4th letter. For example with the word “incorporate”, there are no extra letters Later on and thus has to be the word "increase". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. In the https://fellowfavorite.com/story20205942/the-invalid-bip39-phrase-diaries