For words that only have 3 letters, there is no 4th letter. For example with the word “increase”, there aren't any additional letters Later on and thus should be the word "insert". The word “addict” is during the BIP39 wordlist but you would've made use of “addi” for “addict”. Stability: https://indexedbookmarks.com/story18963214/not-known-facts-about-bip39-english-words