For words that only have 3 letters, there is not any 4th letter. As an example with the word “include”, there won't be any additional letters afterwards and therefore must be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've made use of “addi” https://sound-social.com/story9775146/an-unbiased-view-of-bip39-seed-phrase-github