Principles of operation:
The 4-digit 7-segment LED display module
has 12 pins, 8 pins used for segments (anodes),
and 4 used for common cathodes.
4 common cathodes used to select which digit to display.

The micro controller selects digits by selecting one digit at a time by setting one common anode low, and switching digits rapidly.
When digits repeated at interval every 30 ms or less, the display appears to be continuous.

Control of the 4-digit 7-segment display is fully programmatic by Arduino.
The microprocessor repeatedly selects one digit by setting
common cathode pin  LOW.
Then displays digit by setting HIGH segments that need to be lit.
The program has table which of 7 segments to lit for each digit.
Then waits delay about 6 ms or less, then repeats the process for next digit.

The 7-segment display can show not only digits but also some letters and 
signs and even non-letter symbols, because microprocessor can lit any 
segments in arbitrary combination.
Non-digit symbols and letters can be displayed such as A b d C g F H - E P n S.