easier to just use the built-in hardware counter,
AtTiny2313 has a 16 bit hardware counter, set the limit register to 10000 and you can have a hardware carry output. loop that output back to the input for the other 16 bit counter and you could drive 8 digits, but for the lack of enough I/O pins
the display, (which usually need only be fast enough for human perception) can then be done in software.
so do microcontrollers (to a limited extent)
Bye. Jasen