For words that only have 3 letters, there is no 4th letter. Such as with the word “increase”, there isn't any a lot more letters afterwards and thus needs to be the word "insert". The word “addict” is from the BIP39 wordlist however , you would've employed “addi” for “addict”. https://listbell.com/story8895182/the-definitive-guide-to-bip39-seed-phrase