For words that have only 3 letters, there's no 4th letter. By way of example with the word “increase”, there won't be any more letters afterwards and thus should be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would've made use of “addi” for https://mylittlebookmark.com/story4781592/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting