For words that have only 3 letters, there is not any 4th letter. For example with the word “include”, there isn't any much more letters Later on and so has to be the word "add". The word “addict” is in the BIP39 wordlist however you would've applied “addi” for “addict”. https://bookmarklinx.com/story18919221/ledger-seed-phrase-bip39-no-further-a-mystery