For words that only have 3 letters, there is no 4th letter. For instance with the word “increase”, there isn't any much more letters afterwards and so has to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've utilized “addi” for “addict”. You have https://directmysocial.com/story3655090/bip39-seed-phrase-fundamentals-explained