You are right that every word can be just stored as a four letter word and the system will know it immediately or at least you will but it is important to note that some wallets may not use BIP39 therefore if you use this way of storage it might be dangerous as it might be too ambiguous with only just four letters. The explanation for this is because Electrum launched first before BIP39 so while they now use the BIP39 standard, electrum seeds are not BIP39 exactly. Though I am pretty certain that a lot of wallets uses BIP39 now but just to be safe, still check if you want to store your seed phrase only through 4 letter words.