For words that only have 3 letters, there isn't any 4th letter. One example is with the word “add”, there are no more letters Later on and so have to be the word "add". The word “addict” is in the BIP39 wordlist however you would have used “addi” for “addict”. https://kbookmarking.com/story19455327/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition