For words that only have 3 letters, there is no 4th letter. For example with the word “increase”, there isn't any additional letters afterwards and so should be the word "add". The word “addict” is in the BIP39 wordlist however you would have made use of “addi” for “addict”. A https://socialfactories.com/story4495090/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting