For words that only have 3 letters, there's no 4th letter. For instance with the word “increase”, there won't be any more letters Later on and so needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would have made use of “addi” https://trackbookmark.com/story20278332/the-5-second-trick-for-ledger-seed-phrase-bip39