For words that have only 3 letters, there isn't any 4th letter. As an example with the word “increase”, there isn't any more letters Later on and so have to be the word "increase". The word “addict” is inside the BIP39 wordlist but you would have made use of “addi” https://royalbookmarking.com/story19593899/an-unbiased-view-of-bip39-seed-phrase-github