For words that have only 3 letters, there isn't any 4th letter. Such as with the word “insert”, there aren't any more letters Later on and so have to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have applied “addi” for “addict”. https://bookmarkplaces.com/story18748617/5-simple-techniques-for-bip39-seed-phrase