For words that have only 3 letters, there is no 4th letter. For example with the word “insert”, there aren't any a lot more letters afterwards and thus have to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've made use of “addi” for https://bookmarklogin.com/story18922180/the-5-second-trick-for-ledger-seed-phrase-bip39