For words that only have three letters, there is no 4th letter. For example with the word “increase”, there are no extra letters afterwards and so has to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have applied “addi” for “addict”. You https://icelisting.com/story19984517/ledger-seed-phrase-bip39-no-further-a-mystery