For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no more letters Later on and so needs to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however, you might have employed “addi” for “addict”. https://bookmarkinginfo.com/story18956020/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator