For words that only have three letters, there is absolutely no 4th letter. As an example with the word “increase”, there isn't any additional letters afterwards and therefore must be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have made use of “addi” https://bookmarkity.com/story19030639/bip39-seed-phrase-mnemonics-generator-an-overview