For words that have only three letters, there isn't a 4th letter. By way of example with the word “include”, there are no additional letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however you might have used “addi” for “addict”. https://ragingbookmarks.com/story18822894/new-step-by-step-map-for-ledger-seed-phrase-bip39