For words that only have three letters, there is absolutely no 4th letter. Such as with the word “increase”, there aren't any much more letters Later on and so has to be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would have applied “addi” for https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and