For words that only have 3 letters, there's no 4th letter. As an example with the word “increase”, there won't be any much more letters afterwards and so has to be the word "add". The word “addict” is in the BIP39 wordlist however, you would've utilised “addi” for “addict”. This https://socialmphl.com/story20761861/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting