I am currently looking for new speakers for my 2004 pontiac grand am gt i have already replaced the old stareo, but with the 4 way system im not sure if i can just throw any matching set of speakers

Go for the sound and brand you like.

Impedance is the key. If you go wrong on that, it may damage the stereo. The impedance should be given for any speaker, usually on the packaging or printed on the rear of the speaker. Also, ensure that you hook up the wires correctly, there is a "hot" and "ground" just as with any electrical current.