Will a 4 ohm amp work with a 8 ohm receiver?

Yes, if you use two 4 ohm speakers or or one 4 ohm resistor and a 4 ohm speaker to equal the 8 ohm resistance per channel. An important thing to note about this, is that you will need to make sure you get a resistor that can handle the current. You can go higher in resistance but not lower, it must be equal to or higher. Something important is added: The amplifier will have an output impedance of around 0.04 ohms. In hi-fi we have always impedance bridging. Zout << Zin. The damping factor DF = Zin / Zout tells you what Zout is. Zout = Zin/DF. If the damping factor DF = 200 and the loudspeaker impedance is Zin = 8 ohms, the output impedance of the amplifier is Zout = 8 / 200 = 0.04 ohms. You see, there is no "8 ohm amplifier" with a 8 ohm output. Scroll down to related links and look at "Voltage Bridging - Zout << Zin".