For words that have only 3 letters, there is no 4th letter. Such as with the word “add”, there isn't any much more letters afterwards and therefore have to be the word "add". The word “addict” is in the BIP39 wordlist but you might have employed “addi” for “addict”. How https://bookmarkmiracle.com/story20375792/5-essential-elements-for-bip39-explained