I've uploaded another long how-to blog post. This time I've cascaded four 74HC595 shift registers to drive 4 7-segment displays from only 3 Arduino lines. I also added code to take the number to display from serial port input.
My blog post has instructions, a full schematic of the circuit, some photos, and a YouTube video link. Check it out here.
Hopefully someone finds this interesting/useful - hard to tell with no feedback.