The "loop()" method is repeated over and over until the arduino is turned off. For each loop you are clearing the display and writing the same text to it. You should start by modifying your code so that the display is only cleared and written to when necessary. When you've got that sorted, you can continue with actually reading the code / number sequence and store it in memory (char codeBuffer[4]) - but in order to do this, you would need to learn how to read the state of a button and how to handle debounce 
Oh, and: The "setup()" method is only executed once during startup - reading the button pins here makes no sense since their state may change during the "loop()" phase of your sketch