For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any more letters afterwards and therefore need to be the word "insert". The word “addict” is from the BIP39 wordlist however , you might have applied “addi” for https://onelifesocial.com/story4147683/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator