For words that only have 3 letters, there is no 4th letter. As an example with the word “increase”, there are no additional letters afterwards and so need to be the word "increase". The word “addict” is while in the BIP39 wordlist but you would've applied “addi” for “addict”. Stability: https://bookmarkpagerank.com/story19489025/ledger-seed-phrase-bip39-no-further-a-mystery