For words that have only 3 letters, there is no 4th letter. By way of example with the word “include”, there won't be any much more letters afterwards and thus have to be the word "increase". The word “addict” is in the BIP39 wordlist however , you would've applied “addi” https://bookmarkeasier.com/story18680712/ledger-seed-phrase-bip39-no-further-a-mystery