For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “insert”, there won't be any far more letters afterwards and so has to be the word "increase". The word “addict” is while in the BIP39 wordlist however, you would've made use of https://allyourbookmarks.com/story18841257/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-seed-phrase-mnemonics-generator