This is a conventional 7 segment version of 6 segment 2 digit LED display board, designed for a normal display experience. It is cascadable in the same way, and the notes for the 6 segment board also apply, except that it has segments a-g instead of a-f, of course.

Note that although I have received the fabricated boards, I have not populated them yet as I am waiting for a bulk purchase of LEDs to arrive. Each digit requires 28 3mm LEDs and the colon 4, so for 4 digits 116 LEDs are needed. I don't expect problems with the circuit design since it's the same as that for the 6 segment board. When I have successfully verified one board I will mark this project as completed.