For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any more letters Later on and therefore must be the word "increase". The word “addict” is during the BIP39 wordlist but you might have utilised “addi” for “addict”. https://socialdummies.com/story3919058/5-simple-techniques-for-bip39-seed-phrase