If they suggest 6 to 8 ohms and do not give an output figure for 4 ohms I would probably not use them; I would check with them to be sure.
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Receivers have a LOT of stuff crammed into a tight space. heat is the enemy, and 4 ohms creates a LOT of heat. You would be best off NOT using it with 4 ohm speakers.
However, if you only play at low to middle volumes and NEVER loud, you could maybe get away with it. But the first time you crank it up.. you may blow it up.