2000 Sunfire sound system help

ok, i will start by saying i know next to nothing about cars. i am that stereotypical girl that only knows "cars go vroom!" my boyfriend has a 2-door Pontiac Sunfire 2000 that he bought used with rear speakers that need replacing. he purchased 2 6x9 speakers (here's where there's lingo i dont know a lot about) that are 4 ohm and he read that the Monsoon stereo in his car (i believe it's all original... same speakers, cd player, etc) is 8ohm, and that's what the speakers say, too. so, my question is will the 4ohm speakers work? he said something about needing to find the amp, but doesn't know where it's located.