Countdown timer is probably the most natural project for the 4-digit 7-segment display. While exploring what is available online I found several projects, but did not find the one that I had in mind, namely, a standalone adjustable countdown timer that you can conveniently adjust with the buttons (and not by uploading a modified code). So I decided to make one.
I spent some time to write a code that will make it easy to use and adjust the timer, so that it can be used in practice.
The included code also can be used in other projects with 4-digit 7-segment display. In particular, it has the function that displays a given number (0-9999) for a given time interval. Maybe this functionality is in some of the libraries, but a quick search did not reveal it. Since programming this display is a bit tricky (as you can show only one digit at a time), such a function is really handy.