NOO if you put it there its gonna explode! its gonna burn your entire house, its gonna melt your face! everything you love will be destroyed.
not... im kidding.
if i recall correctly the 4 extra pin on a 8+4 ATX form is mostly used when you need extra power for overclock and all, it seems that using only the 8 pin will be fine if you dont try to push the limits of things and then, obviously you'll be lacking power and everything will shut down or crash or something, ideally you should use a proper psu, but in theory you can get away with it using stock settings.