For words that have only 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any a lot more letters afterwards and thus should be the word "increase". The word “addict” is from the BIP39 wordlist however you would have used “addi” for “addict”. https://bookmarkproduct.com/story19701428/ledger-seed-phrase-bip39-no-further-a-mystery