For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “insert”, there won't be any extra letters afterwards and so should be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've made use of “addi” https://ez-bookmarking.com/story18780217/invalid-bip39-phrase-things-to-know-before-you-buy