For words that have only three letters, there's no 4th letter. As an example with the word “insert”, there aren't any additional letters afterwards and so should be the word "add". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for “addict”. This short article https://yesbookmarks.com/story18899262/an-unbiased-view-of-bip39-seed-phrase-github