- Take a board slightly wider than the tape and approximately 1" thick, and long enough to manageably slide the paper tape across it (the long way).
- Attach a second board against the thickness to make a fence for the paper to slide against and stay in alignment.
- Drill 9 holes across the width of the board with the same spacing as the holes in the tape; mount IR transistors or photoresistors in them pointing upwards.
- Do what retrolefty said, except I think a bright lamp pointing down on the board would be sufficient.
- Calibrate your reader by sliding the paper tape across the board while watching the analogRead values and make sure you get decent variance in the reading as to when the detector sees a hole or not.
- Write your program to take a reading each time the sprocket hole passes over your 4th detector.
Really doesn't seem like that hard of a problem.