If the car idles at 4000 rpms, something is definitely wrong. Most likely it's a major vacuum leak, possibly a sticking throttle cable--but that's really unlikely. Most vacuum leaks happen because a hose breaks or comes loose at one end. Most vacuum hoses are small, and will only cause the idle to go up to about 2500, so you either have more than one vac hose loose, or it's one of the big vacuum hoses (brake booster, PCV). Or you've developed a major leak in your intake manifold gasket.
Open the hood with the engine running, and listen for something hissing. A piece of rubber hose in one ear can help you find where the hissing is coming from. Be careful of fans and belts, don't hurt yourself.
Also (I almost forgot because it's pretty obvious) if your check engine light is on, take the car to an auto parts retailer and get them to scan the codes. If the problem is a bad sensor (it would almost have to be the TPS, to do this) it will have a trouble code stored in it's memory.
