For words that only have 3 letters, there is no 4th letter. For instance with the word “increase”, there isn't any a lot more letters Later on and so needs to be the word "insert". The word “addict” is in the BIP39 wordlist however, you would've utilised “addi” for “addict”. https://bookmarkinglive.com/story19620863/the-5-second-trick-for-ledger-seed-phrase-bip39