For words that have only 3 letters, there is absolutely no 4th letter. For example with the word “increase”, there aren't any a lot more letters afterwards and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however you would have utilised “addi” for “addict”. https://pr8bookmarks.com/story18928612/ledger-seed-phrase-bip39-no-further-a-mystery