For words that only have three letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any additional letters afterwards and so have to be the word "add". The word “addict” is while in the BIP39 wordlist however you might have made use of “addi” https://zenwriting.net/keenanctdp/h1-b-unlocking-the-power-of-bip39-a-comprehensive-look-at-word-lists-and