Re: 4-Digit 7-Segment LED
Well, it's got 12 pins, so it is almost certainly a multiplexed 4digit * 8 segment (including the decimal point) display. Since you ONLY have 12 pins, it's pretty easy to figure out the wiring using a battery, a 1k resistor, and a couple of alligator clips.
Basically, "assume common anode" and connect the + side of the battery to one of the pins on display, and use the resistor and other clip to probe the other pins. You should get one of three behaviors:
1) nothing lights up. This means that the the display is actually common cathode, and you have the + clip connected to one of the cathodes. Reverse the clips on the battery and try again (you'd then be assuming common cathode, and you'll jump to step 3 since you already know a cathod.)
2) Most pins do nothing, but 4 pins light up a (same) segment in each of the 4 displays. This means that the clip is connected to that segment anode pin, and each of the 4 probed pins is the common cathode for the respective digit. Write those down on paper (pin N = cathod of digit 1, etc), this proves that the display is common cathode, and gives you one segment anode pin and all 4 cathode pins. Reverse the batter clips and put the - clip on one of your common cathodes.
3) if you have the + connected to a common anode (or the - connected to a common cathod), you should see each segment of a particular digit light up when you probe 8 of the other pins (and the 3 pins that don't do anything are the the other common anodes (or cathodes.) Mark down which pins are which segments, and which are the other "commons".
(this is easier to do than it is to describe. I wish I had Lady Ada's video skills! You should wind up with a "these pins are the +/- common for each digit, and these pins are the individual segments for each digit.) From there, it's ready to connect up to an arduino in any number of fashions; search for "multiplexed display"