That 100 w / channel @ 8 ohm spec is almost meaningless. Kind of like if I told you I can type 200 words per minute, but didn't qualify that statement with what kind of accuracy I can achieve. Would you rather hire Typist A, who types 100 words per minute with 97% accuracy, or Typist B who types 200 with a 45% accuracy?
So your receiver might achieve 100 watts at only 1,000 Hz, and with 2% distortion. It'd be better to know that it achieves 100 watts from 20 Hz to 20,000 Hz, with never more than .05 % distortion.
More to the point of your question, ideally they'd tell you how the amp will perform at 4 or 6 ohm speakers. I've read that the 4 ohm rating should be 50% or more higher than the 8 ohm rating. The resistance of your new 4 ohms will be much less, so it should draw way more watts. If it can't, then the amp is being severely stressed out.
This will be more of a problem if the new speakers are of low sensitivity, than if they are higher (like more than 92 dB).
Why are you using car audio woofers? You say you are replaciing speakers with woofers...Do they include tweeters?