For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “increase”, there are no more letters afterwards and therefore must be the word "insert". The word “addict” is in the BIP39 wordlist however, you might have used “addi” for “addict”. Mnemonic https://bookmarksfocus.com/story4399295/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator