For words that only have 3 letters, there is no 4th letter. By way of example with the word “increase”, there aren't any more letters afterwards and thus has to be the word "include". The word “addict” is from the BIP39 wordlist but you would've utilised “addi” for “addict”. This https://bookmarkinglog.com/story19475014/an-unbiased-view-of-bip39-seed-phrase-github