For words that have only three letters, there is absolutely no 4th letter. For example with the word “increase”, there isn't any far more letters afterwards and therefore needs to be the word "add". The word “addict” is during the BIP39 wordlist but you might have used “addi” for “addict”. https://bookmarkproduct.com/story18920507/an-unbiased-view-of-bip39-seed-phrase-github