For words that only have 3 letters, there isn't a 4th letter. Such as with the word “increase”, there aren't any additional letters afterwards and so should be the word "increase". The word “addict” is within the BIP39 wordlist however you would have made use of “addi” for “addict”. Mnemonic https://cruxbookmarks.com/story18867243/little-known-facts-about-bip39-seed-phrase-list