For words that only have three letters, there's no 4th letter. For example with the word “add”, there isn't any additional letters Later on and thus must be the word "add". The word “addict” is from the BIP39 wordlist but you might have made use of “addi” for “addict”. A https://bookmarklinking.com/story5382389/the-2-minute-rule-for-bip39-all-words