For words that have only 3 letters, there is absolutely no 4th letter. For example with the word “increase”, there won't be any extra letters afterwards and therefore has to be the word "include". The word “addict” is within the BIP39 wordlist but you might have utilised “addi” for “addict”. https://hotbookmarkings.com/story19486719/bip39-seed-phrase-mnemonics-generator-an-overview