Quick question about power cables, 4 wires...Red1, Black2, Black3, Yellow4.
1 & 2 complete the circuit and 3 & 4 complete another circuit. I know more voltage flows through 3 & 4. I just wanted to know if someone knew the voltage across those to those 2 sets of points. I have 3 case fans, one originally came crimped into 1 & 2, but im going to splice into 3 & 4 because the other to are that way and they push alot more air. Also i did a test with a christmas light bulb and put it through 1 & 2 and it lit up like normal, 3 & 4 and it was blinding me.

Red=+5v
Yellow=+12v
Black=ground

I would advise against powering a 5V fan on 12V. It may be destroyed instantly, or it might break down in a matter of hours or days. Yes, it will spin faster, but you might ruin the fan in the process or greatly accelerate it's death.

Thanks for the info. Another question i have now, is it possible to run the 12V and 5V in parallel or series to the fan? or will that short circuit something. Im guessing there separate circuits and the 12V and 5V cant be hooked up together. 17V would be nice though.

Technically yes you can put them in series and use a single ground and get 17V, however there is no practical use for it unless you get some DC16V fans.

Well I spliced the one fan into the 12V circuit. After reading the fan sticker it says 12V so its safe, i dont know why it was crimped into 5V it was spining slow i could see the blades still, now its running like it should. Thanks for your help.

If you actually wanted to use low voltage for your 12V fans, bridging 12V and 5V for the differential isn't good. Many 5V power rated electrolytic/tantalum capacitors only have 10V breakdown rating.

Here's a better low voltage usage method for your 12V fans.