For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “add”, there isn't any additional letters afterwards and so has to be the word "increase". The word “addict” is from the BIP39 wordlist but you might have utilised “addi” for “addict”. https://sociallweb.com/story4257090/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator