For words that only have 3 letters, there's no 4th letter. By way of example with the word “increase”, there are no additional letters afterwards and thus need to be the word "insert". The word “addict” is in the BIP39 wordlist however you would have applied “addi” for “addict”. By https://allbookmarking.com/story19243377/facts-about-bip39-seed-phrase-word-list-revealed