For words that only have three letters, there is absolutely no 4th letter. Such as with the word “increase”, there aren't any more letters afterwards and thus needs to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would've utilised “addi” for “addict”. A further https://techonpage.com/story4197094/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting