For words that only have three letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any more letters Later on and so has to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. https://mirrorbookmarks.com/story18792431/an-unbiased-view-of-bip39-seed-phrase-github