I know exactly what to do! Seriously!
Ok, what you do, is you download RedSn0w. Once you got that, go on google, and look up, "iPhone 4.2.1 firmware." You can download that from somewhere. Once you have that, open RedSn0w, and do what it tells you. when you get to the step that tells you to select a software, select the 4.2.1 firmware. It'll do it all for you from there. DO WHAT IT SAYS EXACTLY. If you do, you'll be in good shape.
It'll ask you if you want Cidia on your iPhone or not, and a bunch of other options. Select or unselect the ones you want and you're good to go!