The HDP6 is rated at 4 ohms but my receiver has speaker outputs listed as 8 ohms. Will the HDP6s work with my receiver?

The lower the speaker impedance, the more current (amperage) your amp or receiver will need to provide, but most separate power amps have more than a sufficient power supply and power amp sections for driving 4 ohm loads with ease. In fact many amps will perform better with a 4 ohm load as opposed to 8 ohms if their design is robust enough to supply the extra current. A quick web search will bring up more information on this subject.