For words that have only 3 letters, there is no 4th letter. As an example with the word “increase”, there won't be any much more letters Later on and so should be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have applied “addi” https://keybookmarks.com/story18942789/the-2-minute-rule-for-bip39-all-words