For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “include”, there are no a lot more letters Later on and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you https://webcastlist.com/story20170102/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition