For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “increase”, there aren't any a lot more letters afterwards and thus has to be the word "add". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. https://mysocialquiz.com/story4280113/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator