For words that have only three letters, there is no 4th letter. By way of example with the word “incorporate”, there aren't any additional letters Later on and so has to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've made use of “addi” https://fatallisto.com/story8791800/considerations-to-know-about-bip39-seed-phrase-generator