For words that only have three letters, there isn't a 4th letter. As an example with the word “increase”, there won't be any much more letters Later on and thus have to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” https://followbookmarks.com/story18898611/ledger-seed-phrase-bip39-no-further-a-mystery