For words that have only 3 letters, there is no 4th letter. Such as with the word “insert”, there aren't any more letters afterwards and therefore has to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have utilised “addi” for “addict”. Standard https://cheapbookmarking.com/story18755216/the-2-minute-rule-for-bip39-all-words