For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “increase”, there isn't any more letters Later on and therefore has to be the word "add". The word “addict” is while in the BIP39 wordlist however , you would have employed “addi” https://friendlybookmark.com/story18707491/new-step-by-step-map-for-bip39-english-words