For words that have only three letters, there isn't any 4th letter. One example is with the word “increase”, there isn't any additional letters Later on and therefore should be the word "add". The word “addict” is while in the BIP39 wordlist however you would have made use of “addi” https://thekiwisocial.com/story4959423/bip39-seed-phrase-mnemonics-generator-an-overview