For words that have only three letters, there is absolutely no 4th letter. For instance with the word “increase”, there are no much more letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist however, you would have made use of “addi” https://sociallytraffic.com/story3750001/getting-my-bip39-words-to-numbers-to-work