For words that only have 3 letters, there's no 4th letter. By way of example with the word “insert”, there isn't any more letters Later on and therefore have to be the word "increase". The word “addict” is within the BIP39 wordlist but you would've used “addi” for “addict”. One https://bookmarkingbay.com/story18897376/new-step-by-step-map-for-ledger-seed-phrase-bip39