For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “incorporate”, there aren't any much more letters afterwards and thus must be the word "increase". The word “addict” is within the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmarkpressure.com/story18918118/an-unbiased-view-of-bip39-seed-phrase-github