Programmable four digit PIN lock

I have made a four digit PIN lock before, even with the possibility to open two different doors each with its own code, but not with the possibility to program the code. When I saw this I was inspired to make one myself. But I used a binary (4-bit) way of entering and remembering each digit.

I won't go in to deep in to how it works. But I will tell you how I made one bit of memory. I used one vertical XOR gate (I used a modified design of reconJef) and connected one of the two XOR inputs directly to the output of the buttons and the other input is indirect connected to that output, because I placed a D Flip-Flop between them. How this looks like when build see the circuit image, where A is the signal input, B the output (is low when the signal is equal to the memory) and C the input to change the state of the memory (when high it will take over the state of A). The reason why I used XOR gates is that if all the four bits are correct all the XOR outputs will be low and because all these outputs are connected to each other only then the common state will be low (and when inverted -> high).

To enter a code press four buttons (after a button is pressed a lamp will turn on in the ceiling to indicate at which digit you are) and then stand on the pressure plate with the sign that says "Enter" next to it. The current code is 1337. To change the code enter the current code, but instead of standing on the pressure plate, press the button on the wall on you right and then enter the new code.

Entering and programming codes are done, but I am not jet sure how I would do the user interface. For now I also only have an extending piston to indicate that there is a high signal which could be used to open a door.
I am open for suggestions for a user interface, such as a counter which would count how many times someone has entered a wrong code. So if you have an idea how you would like hoe it would work, let me know!

Additional Notes

I used MCedit, to many items and single player command mod to build this.