For words that have only three letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any more letters Later on and thus need to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have used “addi” https://bookmarkangaroo.com/story18918514/bip39-phrase-generator-can-be-fun-for-anyone