Electrical – 7 Segment LED – What is the difference between driving 1-digit and driving 4-digits

Actually this is a very common arrangement. Each digit has its anodes connected together, and the segments for all digits have their cathodes connected. The controller enables one of the common anodes, drives the segments, and waits, then turns off the cathodes and moves on to the next common anode. This arrangement and control method is called multiplexing, and the linked article will give you a lot of information. The part in question is a common anode display, but common cathode displays are just as common.

Multiplexing allows you to drive a large number of display elements with fewer wires than individual addressing, at the expense of a more complex control algorithm. You can go a step further by doing something called Charlieplexing which allows you to drive even more elements with less I/O if you can turn off the drivers (as opposed to just setting them high or low, which is the requirement for a multiplexed display controller).

Now that you know it's called a multiplexed display, you should have no trouble finding all kinds of example circuits or microcontroller code fragments to implement it.

According to your schematic, you have the outputs of all counters connected in parallel, and to the anode inputs of your display. You have all cathode leads of the display grounded. This will force all digits to display the same number.

The display you have needs a multiplexed input. You will need seven 4:1 multiplexers between the counters and the display, with the segment outputs of counter 1 going to input 1 of the multiplexers, counter 2 to input 2, etc. You also need to arrange to ground one cathode lead of the display at a time, switching the mux and cathode selection in sync, so that display 1 will be enabled when the mux selects counter 1.