For words that only have three letters, there's no 4th letter. By way of example with the word “increase”, there won't be any more letters Later on and so have to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would've made use of “addi” https://mysocialport.com/story4392049/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting