Impedance is not a constant thing. It varies quite a bit with frequency. The impedance of an 8 ohm speaker can go up to 40 or go down below 5. Most high-end receivers should handle loads above 6 ohm without difficulty. However, I would hesitate to use a receiver with 4 ohm speakers. That is one reason we have separate amplifiers that can handle low impedance speakers. Please read the thread on
Impedance & Sensitivty of a Speaker. It has sticky under Speaker Discussion.