Sorry I don't know the proper term but it's the deafening sound just like you point the microphone toward the speakers.

Anyway, my 4ohms speakers have this problem when I karaoke with the volume fairly loud. When I switch to 8ohms speakers the problem went away.

Could someone explain why and if any damages done to the speakers or receiver/amp when it happens?
Its called feedback. If everything is playing okay, surely you are in the clear. Except for maybe your ears :), any damage is going to be acute.
But why would it happened for the 4-ohm but not 8-ohm speakers at the same volume?
Just a guess, but it might be the pattern your speakers broadcast rather than their resistance that's making the differance. I work with some proaudio and have seen all tweeters blown at the same time, it only took a second. Try aiming your speakers such that the mic is always behind them.
Woodman is right. The feedback is caused by having the microphone in front of the speakers.
Thanks everyone for inputs and suggestions. I guess I'm stuck with what I have since this is a family room setup so mic can't be behind speakers.