There's no such thing as a 4 ohm speaker, or an 8 ohm receiver, or a 3/4-ton pickup truck. The terms are nebulous, and they omit many other variables.
A speaker rated at 4 ohms may have resistance at various frequencies varying from 4 ohms to 20 ohms, or higher. Some 4 ohm speakers don't dip below 5 ohms at any frequency, but others dip to 3.2 ohms or lower, and they can cause amplifier problems and current limiting. Then there's the question of where the low resistance occurs. If it's for an LCR switched to "small" and the impedance is 3 ohms at 30 Hz, no big deal. Also, what is the speaker's sensitivity? If it's high, the receiver may not be pushed hard enough to draw enough current through the outputs to shut it down.
You can generally drive a 4 ohm speaker with a receiver not rated to drive below an 8 ohm load (I hate those, BTW), as long as you don't turn the volume up too toasty, and as long as you add NO treble or bass boost. It's living on the edge, though.
aka TRIAD DUDE
I can tell people who are judgmental just by looking at them...