For words that only have three letters, there isn't any 4th letter. For example with the word “insert”, there aren't any a lot more letters afterwards and therefore must be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have made use of “addi” https://allkindsofsocial.com/story4123754/a-review-of-bip39-phrase-list