You don't really need all that glue logic. Just some transistors to drive the common cathode of the displays. With 12 IO pins you can do it all without any extra chips. The 74LS154 is not doing much for you as it only saves you one IO pin.
hi kchiste,
thank you for studying my circuit.
I planning to use/buy pic16f628 and use RA ports, then other will be reserve(for future) especially Tx/RX port (connect to Pc) and also planning to extend the digit. I think this will save internal memory and lesser conversion from binary to decimal .