the 4th pin is just a controller (voltage or speed), sometime motherboards have them some don't, some can control it with software, its not too important, just means you prob can't control the speed. So yes you can just plug 4pin into 3pin
Why aren't you using the CPU fan plug? its good if your fan fails your computer will know to warn you.
I'm not sure if you can do this on all board but some boards (prob most) you can turn off the error messages (for if your using liquid cooling) in the BIOS.