For words that have only three letters, there is not any 4th letter. As an example with the word “incorporate”, there isn't any extra letters afterwards and thus has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you might have employed “addi” for “addict”. https://gorillasocialwork.com/story21887253/ledger-seed-phrase-bip39-no-further-a-mystery