simultaneously count with all sensor to a 12 units in total (not each)
I very much doubt if you need to read 4 inputs simultaneously, which you can do if you choose the right 4 pins, rather you can read each of the 4 pins sequentially so fast that to all intents and purposes they have been read at teh same time, certainly in human perceivable timeframes.
What have you tried so far ?
Can you read the state of one input and increment the value of a variable when the input changes state ? Note the need to detect the change of state rather than simply its current state to avoid multiple counting.
UKHeliBob:
What type of sensors will you be using ?
Those yellow cigar things look like ir to me, so should as you say be able to count using state change detect, high to low transition probably.
And should also work even if those screws (if those are the items to be counted) hit the sensors literally at the same time, since they should stay in front of each sensor long enough for loop() to get round and count the next sensor.
