For words that only have three letters, there is absolutely no 4th letter. One example is with the word “include”, there are no much more letters afterwards and therefore have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would have used “addi” https://onlybookmarkings.com/story18882190/ledger-seed-phrase-bip39-no-further-a-mystery