For words that have only three letters, there's no 4th letter. For instance with the word “add”, there isn't any more letters Later on and therefore must be the word "insert". The word “addict” is within the BIP39 wordlist but you might have made use of “addi” for “addict”. This https://getidealist.com/story20590502/the-greatest-guide-to-seed-phrase-bip39