For words that only have 3 letters, there isn't a 4th letter. For example with the word “increase”, there aren't any more letters afterwards and thus needs to be the word "insert". The word “addict” is in the BIP39 wordlist however, you would've made use of “addi” for “addict”. You https://dirstop.com/story22504282/facts-about-bip39-seed-phrase-github-revealed