For words that only have 3 letters, there's no 4th letter. Such as with the word “insert”, there won't be any far more letters afterwards and thus should be the word "incorporate". The word “addict” is while in the BIP39 wordlist but you would've used “addi” for “addict”. This post https://indexedbookmarks.com/story18782420/fascination-about-bip39-english-words