Many people enjoy the sound of vintage receivers, and there are MANY that can drive 4 ohm speakers. Almost all the Marantz and Pioneer receivers from the 70's can drive 4 ohm speakers, (and were rated for such use) and if you go back to the 60's you can get yourself a classic tubed receiver like my Fisher 500c. A decent example of which can be had within your price range.

As to Marantz I would start looking at a 2270 model and go up from there. With Pioneer your range should start looking at the SX950 level. Both these often come up on Ebay.
If you can snag a Marantz 2325, or Pioneer SX1250 in good condition for under $500 your in for a treat. Either of these receivers will shame most modern recievers in both sound quality and shear power.