Don't get too hung up on specs. You can check their DC resistance with a multimeter if you wish, which will give you a general idea of their AC impedance. Even if they're 4-ohm speakers, they're probably only 4 ohms for a very limited range of frequencies. You still might ought to feel the top of your receiver from time to time, just to make sure it's not abnormally hot. If you can fry an egg on it, back off the volume. And you'll probably rarely if ever hit the peak wattage even when cranking the volume to slightly uncomfortable levels. Just listen critically and if you hear harshness in the tweeters, back off the volume.