For words that have only three letters, there is not any 4th letter. One example is with the word “increase”, there are no more letters Later on and thus should be the word "add". The word “addict” is within the BIP39 wordlist however you would've made use of “addi” for https://bookmarkingbay.com/story18859273/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition