For words that have only 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any extra letters Later on and thus need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've applied “addi” for “addict”. BIP39 and https://nimmansocial.com/story8594323/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator