For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there aren't any much more letters afterwards and therefore have to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” https://bookmark-group.com/story4741811/examine-this-report-on-bip39-mnemonic-phrase-tool