Can I hook a 2-channel amp up to my car's 4 speakers?

yes you can use it that way, you may get some distorsion to the smaller speakers though. 250 watt/2 =125w per channel/2 equals 62.5 watts per speaker. remember in the future, when using an amp, you want your reciever to be as powerless as possible on the output leads. this will take care of alot of your high noise and high energy humm.