For words that have only three letters, there is no 4th letter. For example with the word “increase”, there are no more letters afterwards and so need to be the word "insert". The word “addict” is within the BIP39 wordlist but you would have used “addi” for “addict”. In this https://sociallawy.com/story9453582/facts-about-bip39-seed-phrase-word-list-revealed