For words that only have three letters, there is not any 4th letter. As an example with the word “increase”, there are no more letters afterwards and thus must be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would have used “addi” for “addict”. https://letsbookmarkit.com/story18950377/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator